Don't get any 6600 but the 6600GT. It's only slightly more expensive and it's a much better card. My 6600GT is giving me 60+FPS with the most demanding games I can find ( Oblivion, Tomb Raider Legend, etc )
I'm guessing you're not using Tomb Raider's 'next gen content' option there then ;)
My 7800 balks at the graphical niceties of Legend. It's playable, but you know it's working damn hard. Either it needs 512MB, or the shaders slow it down. I'm guessing it's the former. I would consider an upgrade myself, but I don't spend that much time playing games on the PC.