never had a ati card. I have always used nvidia.
I currently use a 8800gts 640mb OC card from bfg. runs stuff good.
and you can find them under 150 dollars im sure. I have had mine for about 2 years now, still runs fine. dont see where you would have a bottleneck. (use to have a AMD 2.6 ghz processor and the processor had to be replaced, it was causeing a bottleneck.)
http://www.bfgtech.com/bfgr88640gtseeoce.aspx card site over at bfg.
(heat could cause a bit of a issue, just had a little issue myself, card overheated, still runs fine.)