See http://www.tomshardware.co.uk/radeon-hd-6790-barts-gpu-... for a comparison they are both on. I think 768Mb is good for anything below 1920x1080 at this setting im not sure. If you can just get the 1Gb version, but at the end of the day even the 768Mb version should give around 10% more FPS than the 550ti.
The 6790 is a good card its just in that price bracket when an extra 10-15 bucks is nearly always worth it. Let us know where you are buying it from or what the price is on all cards you may consider and we can let you know the best buy.