Oh right, I'll be playing WoW, Gta IV, dragon age 2, and I kinda wanna start playing at 1080i resolution...
WoW prefers nVidia cards, but Dragon Age 2 has driver issues with nVidia at the highest settings. Still, WoW is playable at decent settings on ATi cards too.
If you want more power than a HD6770, getting a HD6850 makes a lot more sense than the HD6790. It is more powerful, and uses less power. The small price difference is worthwhile.