I really wouldn't go for DDR3 these days.
If you don't need boot screens almost any nVidia and most AMDs will work if meeting power requirements (lower than 225W, but if you read this forum you'll see the everything up to 250W is kind of safe).
If you want to flash your card and have full boot screens, PCIe info and proper frame buffer assigned, HD7950, HD7970, R9 280 and R9 280X are the best you can buy for your MP1,1. Just make sure that they have DVI, HDMI 2xmDP layout.
This all is assuming that you're running Mavericks and up on your MP, not SL or Lion.
Btw Do you think the Msi GTX 960 4GB GDDR5 120 watt will work?
Well, frankly, you don't.
Its nice to have it when troubleshooting things, or installing Win on your Mac (for choosing OS X before installing Bootcamp drivers), but you can always keep GT7300 near for those things.
Some cards (AMD) have minor bugs if used unflashed but its rare (although i had it with HD6850 in FCPX).
Some people need PCI info to be functional for various cards they're using, also not possible with unfleshed card (it shows blank/error).
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And having so much topics about GTX680, 7950/90 and R9 280(X) flashing its fairly easy to do.
Again, if you don't need it than the choice for GPU upgrade on MP1,1 is even easier - almost any card will work.
AFAIK, yes, but you'll need OS X 10.10 as minimum for Maxwell GPUs.
Do you need OS X or will you be using it just for Windows gaming?
Do you have access to any other computer, Mac or PC?
If you'll be using it just for Win gaming, its easy, buy whichever GPU you'd like, put MP's HDD in PC and start Windows installation on it (I've tried 7 Ultimate x64 and 10 Pro x64). On the first reboot in Win install, turn off your PC, put the HDD back to MP, boot it up and let it finish the installation.
For OS X install it can be easy as for Win (+ moving three boot.efi files) or it can be a lot more tricky depending on the first two questions in this post.