Rgr, just make sure you can fit ~10in video card in (the 5850 is about 9.5in)
I am sure others will be along to post their suggestions. The 5770 is cheaper, and runs the game very well. I only start to have problems when I have full shadowing on with max shadow textures. But this ceased when I bought a second 5770 for crossfire. I just found shadowing distracting.