Steven1994,
Most of the talk about needing a second (or third?) power supply to power a graphics card is complete nonsense. It really depends on what you are using/needing the graphics card for.
I have 27 Apple Mac Pro's at the office, and have four Apple Mac Pro's at home, and I've always had THREE graphics cards in several/most of my computers, and dating all the way back to 2008 I was able to buy a 2008 Apple Mac Pro with three Radeon HD 2600XT graphics cards already in it (just added 2 additional graphics cards).
A Radeon HD 2600XT uses about 18.1 watts (while doing 2D work) and while doing 3D rendering under 100% full GPU utilization uses 54.4 watts.
See here: http://www.tomshardware.com/reviews/geforce-radeon-power,2122-6.html
Three Radeon HD 2600XT's normally use about 60 watts of power (while doing 2D work) and even with all three graphics cards (in CrossFire) at 100% GPU utlization, I was hovering around 162 watts of power draw (with all three GPU's maxxed out at 100% GPU utlization).
Newer graphics cards are far more powerful but also have much better power management (and they can shut down the unused GPU cores) and they reduce the power consumpion (on idle GPU/graphics cards) to almost zero, and your 2nd and 3rd graphics cards usually never use above 25 watts of power.
In 2012, decided to upgrade to an Radeon HD 7970 GHz Edition. This is a much faster GPU, and just ONE GPU on one Radeon HD 7970 is far more powerful than all three GPU's using three Radeon HD 2600 XT's.
I also replaced several older Radeon HD 5870's with the latest Radeon HD 7970's because I can do more with one graphics card, then I could with two or three Radeon HD 5870's (while actually using less power).
So now, in most of my machines, I have two Radeon HD 7970's and one Radeon HD 2600XT. The Radeon HD 2600XT's were just leftover graphics cards from the older 2008 and 2009 Apple Mac Pro's. I put one Radeon HD 7970 (in slot 1 which is a double slot), and have one Radeon HD 7970 in slot 2 (which takes up slots 2 and 3). I have a Radeon HD 2600XT in slot 4. (This gives me the ability to power 12-14 monitors)
In my opinion, the graphics cards are way too close together for proper cooling, and you really need an EATX motherboard (like an EVGA SR-X) with seven PCIe slots, so that you can space out your graphics cards (for proper cooling) or use water cooling (like on most gaming machines), if you are truly interested in building a "gaming machine" (for extreme 3D rendering/gaming).
If you are just looking to power a few extra monitors (while just surfing the web, or doing standard 2D desktop work) then your second and third graphics cards will be sitting completely idle (~18 watts to 25 watts of power consumption for each of your 2nd and 3rd graphics cards in CrossFire mode). The graphics cards will remain cool (and silent) and you won't have any problems. Your first primary graphics card could consume anywhere between 18-25 watts (idle) and 54 watts (maximum) on a Radeon HD 2600XT. On a Radeon HD 5870, you're talking 166 watts (83 watts per graphics card, while idle), and about 340-360 watts (83 watts + 280 watts) with one GPU at 100% utilization. If both GPU's are both running at 100% full utilization (under CrossFire) and fans are blowing at 100% full speed, then you could draw as much as 561 watts (for two Radeon HD 5870's in CrossFire mode).
Your power draw (on BOTH graphics cards will probably be under 200Watts combined).
See here: http://www.tomshardware.com/reviews/radeon-hd-5870,2422-20.html
If you truly believe that you'll be running your GPU's (in CrossFire mode) at 100% GPU utilization (and trying to start a fire) then sure, get a 2nd power supply.
But even if you bought a second power supply, what is that going to do? Not much of anything, other than start a fire. Truly, if you are maxxing out your 1000Watt Apple Mac Pro power supply, and you feel that you need another (2nd) 450 watt power supply, and truly think that you are using 1450 watts of power, then you are probably going to start an electrical fire in your room (unless you have a dedicated 15 Amp circuit just for your computer and the two power supplies).
Keep in mind that a typical 15 Amp circuit (using 14 gauge wire) can have a maximum draw of 12 amps or 1440 watts. (Never go above 80% of the max watts, so 15 x 120 = 1800 watts x 0.8 = 1440 watts)
If you are going to draw more than 1440 watts (this is including your ceiling lights, computer monitors, speakers, printer, and EVERYTHING in your room and EVERYTHING on that circuit), then you need to re-wire your house/room and put in a dedicated 20Amp circuit (just for your computer) if you truly think you are drawing that much power.
Yes, in server rooms we use dedicated 20Amp circuits for each server (because of the multiple CPU's, and multiple hard drives used in RAID arrays, and often two redundant power supplies). A 20 Amp circuit will give you about 20 x 120 = 2400 watts x 0.8 = 1920 watts (continuous)
Anytime you add a second power supply, you are using/losing more electricity. Most home power supplies are about 80% to 85% efficient. Some server power supplies can be as high as 90% or 95% efficient.
As a safe guess, just figure 1000 Watts on a Mac Pro x 80% efficient, and figure you have about 800 watts of "usuable" power.
That should be sufficient for three graphics cards (doing normal 2D work), and even if you were doing 3D work it's highly unlikely that you would EVER max out the GPU on one single Radeon HD 7970 graphics card. If you were doing something requiring that much rendering power, and ended up using/requiring a second GPU to actually be used, then yes your power requirements would go up a bit.
But normally your GPU's on your second and third graphics cards will be completely idle, and you'll never go above 25 watts (for your 2nd and 3rd graphics cards).
If you truly think you'll be trying to "render the whole world" on your Apple Mac Pro (and doing some SERIOUS 3D GAMING, and you have a nice 16K16K wall of monitors) and you somehow are trying to squeeze every single last frame per second you can out of your computer, then yes... you have a good chance of melting your Apple Mac Pro, and starting an electrical fire in your home).
You may want to consult with an electrician and tell him that you need a dedicated 20Amp circuit just to plug your computer in (for your 1000 watt primary power supply, and 450 watt secondary power supply and UPS and enough power to power all of your monitors/displays).
But in 99% of the cases, if you are just primarily using your monitors/displays for just surfing the web, or doing Microsoft Word, Powerpoint, Excel, or just normal desktop applications (2D) then you are using very minimal power on your graphics cards (especially since your 2nd and 3rd graphics card will always be idle).
I have three graphics cards in nearly all of my 27 Apple Mac Pro's, and I've never run into any problems. I don't have a second power supply in 25 of them, and there are only two Apple Mac Pro's that we have a 2nd power supply in (and those two are used specifically for rendering). Those two have dedicated 20 Amp circuits, and we have them in a room with extra cooling (dedicated A/C unit to keep the room temperature at 50 degrees or lower).
I'm fairly confident that just getting a $6 power adapter from Fry's will be sufficient for what you are trying to do (unless you are a heavy gamer). If you are a heavy gamer, then look into getting a "Juice Box" (450 watt) power supply to power your second graphics card, and also look into getting your house/bedroom wired for a second dedicated 15 Amp circuit just to power your computer (and your two power supplies) because if not, just like The Hatter said:
"You can't get more milk out of that cow than she has,"
You can't squeeze more power/amps out of a circuit than you have. You'll either overload the circuit, pop a circuit breaker, or start an electrical fire.
But two Radeon HD 5870's are nothing. Even two Radeon 7970's are nothing. If you were looking at three GTX 690's or three Radeon HD 7990's (which are dual GPU cards) then yes I would be a bit concerned about power, and yes you'll probably need quite a bit of juice (a dedicated 20 Amp circuit breaker, and an additional 450 watt or possibly even an additional 1000 watt power supply) depending on what you are doing with the graphics cards.