Upgrade video card on early 2009 Mac Pro

Hello!
I have a Mac Pro early 2009 and I want to upgrade the videocard. I currently have a nVidia GT 120 and what is the best card for gaming and photo editing? I have read an ATI Radeon 5870 is good. Will this card work???? If you suggest a better card let me know!

Thanks!
Divetdave

I'd like to know this as well. My Nvidia GeForce just took a crap