The 550Ti is on par with the 6790, therefore it's going to be a little more powerful than the 6770. But i believe that the $125 price after rebates is very good. I personally would go for the 550Ti in this comparison. Especially so when the manufacturer is EVGA.
AMD simply re-branded the 5750 and 5770 as the 6750 and 6770, but the new models tend to cost a little bit more. The 6790 is much like the 5830 - it's a further disabled 6870 than the 6850. Most people think it's a Juniper-based GPU (5770) with a 256-bit bus, but it's a heavily-crippled Barts GPU (6870).