Steven1994,
If you want to get an idea of where you are at EXACTLY as far as power draw, just get a $19 P4400 Kill-a-watt and just plug it into the wall, and then plug your Mac Pro directly into the P4400 Kill-a-watt device, and you can actually see EXACTLY what your electrical draw is.
If you are truly getting anywhere clost to 900+ watts or getting close to 1000+watts draw, then yes I would be a little tiny bit worried, and would look into getting a second power supply but the Apple Mac Pro has a decent/stable power supply in it, and it seems fairly rock solid. Almost all of my Mac Pro's have one single power supply in them. Only two of them have a secondary power supply, and that is only because we use those two machines specifically for 3D rendering. (And yes those machines get quite hot, and we have dedicated 20Amp circuits for each machine, and have them in a separate room/server room which is properly cooled, and we keep the room down around 50 degrees when we are doing heavy rendering). As for what you are doing, which is mostly just 2D desktop work, that second GPU will probably be sitting idle almost all of the time.
I'm guessing that you probably won't even be over 750-800 watts total power draw on your system.
Adding a second power supply is "overkill" in my opinion. Unless you are doing heavy gaming (which you already stated that you are not).
If your power draw is getting relatively high (and you're extremely worried) then just throw in a $24 Juice Box (450 watt power supply). It will mount in your 2nd optical drive bay, and just use that $24 Juice Box to directly power your 2nd graphics card, but I wouldn't think it would be necessary unless you're doing heavy 3D rendering and are somehow using BOTH of your GPU's at 100% utilization.
Take a peek at your GPU utilization, as you're trading, but mine is fairly low.
Also if you're worried about power draw, just get a $19 Kill-a-watt, plug it into the wall and test your system BEFORE you add the second graphics card and then test your computer AFTER you add the second graphics card, and you'll see that there isn't really all that much difference (maybe 100-140 watts?)
I have used Radeon HD 2600XT's simply because they are an older card, but use far less electricity (18 watts). If you just need to power two extra monitors.
Here is a Kill-a-watt device here: http://www.amazon.com/P3-International-P4400-Electricity-Monitor/dp/B00009MDBU
This way you can tell exactly what you are using in terms of watts/amps. If your computer is sitting idle, you are not going to draw that much power. I guarantee that second graphics card will be idle the majority of the time (unless you're doing 3D rendering).