The 6790 rivals the 460 768mb, the 550ti being lower than both. http://www.tomshardware.com/reviews/radeon-hd-6790-barts-gpu-geforce-gtx-460,2917.html
Specific specifications really mean absolutely nothing, it always comes down to fps. Just like in this review, the 6850 uses less power than the 6790 but is almost 20% better.
Some games work better on amd some like nvidia, it depends who the gaming company partners with.
With the rest of your specs, you could handle much higher than a 6790.