A Phenom II works just fine with a 550 TI. You could still go heavier if you wanted. I'd think you could go as high as a 7870 or GTX 570 for sure and not have any complaints.
Now I do have the 550 TI AMP ed, so its overclocked 100mhz over the reference card but, If you have any games specifically in mind, I can bench em if I have em. But off the top of my head, any COD game I can average well over 70FPS @ 1280x1024 fully maxed out AA and all that jazz.
Crysis 2 averages 50-60FPS Ultra settings 1280x1024 again. Skyrim can get a little nasty though, it usually runs okay, but it does indeed dip down to high 20s on occasion. I'm fairly certain is the GPU not the CPU, if I dial the graphic settings down a tad it runs smooth. My boyfriend thinks it runs fine, but honestly, I'm not happy with the dips, I notice it.