Early 2008 8-Core Mac Pro GPU Upgrades

Hi everyone,

This is my first time posting here, so bare with me! Long story short, I have an early 2008 8-Core Mac Pro and I want to upgrade the Graphics Card. I've been reading reviews, comments, critiques, suggestions for a couple weeks now about how the 3,1 Mac Pro along with Mavericks or Yosemite, has the ability to be upgraded with some pretty powerful PC Graphics Cards. (Currently i'm using Mavericks but have no problems upgrading to Yosemite)

That being said, i've now gotten lost in a whirlwind of "which" card would be sufficient for my needs. I am a visual effects artist and definitely push all aspects of a computer to it's maximum. However, a multi-thousand dollar Quadro card is out of my budget. I'd like to stay under 4-5 hundred (less is even better). I currently have the Nvidia Geforce 8800 GT still in use, so it seems that pretty much any of these great sounding upgrades can really do this computer some much needed justice. So my questions are:

Could someone suggest a good card (reasonably priced) that would knock the socks off my computer compared to my current card? I would like to take advantage of OpenCL as well as CUDA. This card will be used primarily for doing all kinds of simulations (destruction, fluids, particles). Also, for rendering purposes. Lastly, not the prime focus of the card, I would like to be able to play some games when time permits!

So, just to put it out there, i've seen that there may be issues with boot screens using the PC based cards - this does not bother me (however if there is some workaround where I can get a boot screen and would be fairly straightforward to follow in terms of directions), I would gladly go for that too. Right now there seems like a great time to purchase a card while there are all the holiday sales. What would you suggest? I saw a GTX 970 FTW on sale for $360 the other day. I'm not a pro at the "statistics" when it comes to graphics cards (in fact this will be my first actual graphics card purchase separate from what comes stock in my computers). Therefore, i'm reaching out to you all for advice.

Thanks for your help.