Right now the 8800GT is just nice for Mac Pro going into, say middle-2009. For hardcore graphics, the Quadro etc. for Mac Pro.
The 9800 or 8800GT's SLI-ed is an interesting proposition, but more for gaming on PC, going into the end of 2008.
I am running HL2: Ep2 and Team Fortress 2, on pretty much Max settings for 1280x1024 with about 4x AA (MSAA or 8xQSuperSAA) etc. etc. 8500GT 256mb OC'ed. Unreal3 on maximum settings.
XP2Pro. Do not touch Vista unless you are seriously enthusiastic about games. 3ghz Quad, dual 8800GTX, 4GB RAM, etc. and a 22" 1900x1200 monitor.
DirectX9.0c and maybe 9.0d(?) on XP2Pro should be fine for me going to the end of 2008.
I will get another 8500GT for SLI, or dual 8800GT 512mb for my PC, if there is a significant need, if say I go up to a 22" 1900x1200 monitor. At which stage I might as well f*king get a PS3.
Ah, the graphics treadmill. That said, I am happy for nVidia's GP GPU push, and buying of Ageia. A fanboy, yes. However, HL2 and Portal has been quite interesting, Unreal3 Engine as well.
Intel quad-core mainstream in 2009, nVidia GPU + GP + Physics, going to be a nice PC gaming scene in 2009. Again, that said, this should be the combination for the next-generation consoles. PS3 and XBOX360 have issues of their PowerPC/Cell (I know I am going to get flamed for saying it)... and Wii of course is the casual gamer scene, maybe a Wii 2 could be what the PS3 should have been?