I didn't check on your PSU, I assume it is a quality unit with 2 pci-e power connectors. XFX is a good brand after all.
The 6850 is slightly faster than the 550 ti, but it also consumes more power. Between those 2, I would be inclined to recommend the 6850...
BUT if you could step up your game to a GTX 560 it will be worth the effort.
I would definitely recommend getting the radeon 6850 against the gtx550 ti as it requires more power than the 6850 but performs less. You probably need extra performance especially as it can be the difference between playable and not playable