i say get a 8600GTS and another gig of ram, but if you have enough money i would say save up for a new build.
you should probably get the 512mb version too, if you want to run it at high resoultion than this is a must. mainly because they use tons of textures.
dont be surpised if it still lags though, even the 8800GTX only gets 23FPS although so do must other cards, its like its capped there on max settings
there is a great chart for you. keep in mind though those benches are with 2gigs of ram and a quad core @ 3.0ghz, there also max settings, so i guess if you cranked down the AA than you would get better frame rates