I'd recommend going with a 5870 and an 850+ PSU. On a single card the 5870 should get the best or near best performance, and the beefier PSU means you can add a second one in the future when they drop in price. That way you don't have to spend a fortune right now and can ensure your build remains valid for several years.
Retired: Inra-Dark Crag 40/40 Witch Elf
Retired: Hollow-Thunderlord 70 Warlock S1/S3 T4 SL/SL
Retired: Horknee-Thunderlord 70 Druid T4/T5 Feral