obDisclaimer: I'm not a harcore PC gamer; the most powerful video card in my apartment is in my Xbox 360, and that's by a very large margin. But I spend way too much time poking around on hardware review sites.
Having said that... pretty much everything in the $150US-$250US range among recent cards (GeForce 8/9 series, Radeon 3800 series) right now is very good from a price/performance standpoint. But there's just not that much drop-off from the 8800GTS (or GTX or the single-GPU 9800 lines) to the 8800GT (and its close cousins, the 8800GTS 512MB and the 9600) except at ultra-high resolutions (above the 1920x1200 you usually see on 24"-27" widescreen LCDs), and that's also about the only place SLI (or CrossFire) is useful.
Having said that... pretty much everything in the $150US-$250US range among recent cards (GeForce 8/9 series, Radeon 3800 series) right now is very good from a price/performance standpoint. But there's just not that much drop-off from the 8800GTS (or GTX or the single-GPU 9800 lines) to the 8800GT (and its close cousins, the 8800GTS 512MB and the 9600) except at ultra-high resolutions (above the 1920x1200 you usually see on 24"-27" widescreen LCDs), and that's also about the only place SLI (or CrossFire) is useful.