See the sig, and for the record from a "playing games" vs "reading benchmarks" perspective, the 5770 is on par. When I first built this rig I had an XFX 4870 1gb installed (the newer one that lacks fan speed control and doesn't vent hot air out the back so I returned it) and I have gamed quite a bit on both cards and found actual gameplay you can't tell any difference. Now I don't play the most demanding games, but WoW:WotLk, Trine, Torchlight, LOTRO, HL2 et al, TF2 Batman AA, Tomb Raider et al, COD4... run perfectly at 1920x1080 4xAA/16xAF details on max (obviously no AA on Batman, thanks nV d-holes). I'm pleasantly suprised how well this combo performs, and we are still 2-3 weeks from official Catalyst support with the Nov release, performance will only get better.
I would not be afraid of the 5770 especially with the amazingly low temps it runs at, how quiet it is and the new features it packs. I'm sorley tempted to go Crossfire and give Eyefinity a whirl, but mostly for WoW and I spend enough on tha game every month so we'll see.