Actually, nvidia aren't the best anymore.

AMD/ATI has the current fastest single-gpu card and is almost always cheaper.
I used to buy nvidia cards, but no more as they are overpriced for lower performance.

Also there's no comparison here, the 6790 is on another level compared to the 550Ti, you'd have to be stupid to buy the 550Ti in this case. So forget what all the fanboys say and get the 6790, it is better, fact.