Just thought I'd chime in...I'm Stepping Up my 8600GTS 256MB for a G92'd 8800GTS 512MB stock version. It should arrive Wednesday. Looking at memory bandwidth alone, I'll be moving from 32GB/s to 62GB/s.
There's just no percentage in purchasing the old G80 cores. I can't believe the price hasn't fallen out from underneath those things. I think that in the right system, an 8800GTS 512 will come close to something like an Ultra. A user-OC'd GTS will definately give those old behemoths a serious run for their money.
If you want my serious advice, take your $700+ a buy two of the new GTS's. Yeah, SLI isn't a great price/performance deal, but if money REALLY isn't an issue...
...then all you have to do is overclock your quad-core straight to hell in order to max out your performance celing, and load up on the RAM.
Or you could just get one 8800GTS and use the money you would have spent on an array of wunder kards to pay your rent.