Remember my quote regarding XFX - "well manufactured, poorly designed". The components used are OK but the design plain sucks. The cooling isn't as good as it should be, they skimp on a few things, etc.
@OP: Look at the prices at techshop and theitwares:
Even considering a rough ballpark estimate, the performance difference between the GTX 550 Ti and HD 6790 is ~5% on average and (HD 6790 is slightly better, that is. The lead becomes dubious with enabled AA and AF though). The price difference, however, is a lot more than that, and the power consumption difference is not that much - a power supply that runs HD 6790 will also run the 550 Ti. You can get an OCed 550 ti (Zotac AMP, EVGA FPB) at 8.1K. Given the choice between an OCed 550 Ti and a regular HD 6790, go for the 550 Ti.
Basically the rough ballpark estimate is like this:
HD 6770>GTS 450
HD 6790 >~ GTX 550 Ti (difference is not big, look for value for money)
HD 6850 >~ GTX 460 (difference is not big, look at value for money)
GTX 460 V2 > HD 6850 (However, the GTX 460 V2 is new and may not be easily found right now)
GTX 560 = HD 6870 (complete tie, get the card that gives maximum value for money, OC edition on the cheap or whatever)
HD 6950 > GTX 560 Ti (no contest, get HD 6950)