I figured some better and more detailed advice would be given, but none so far. Basically, here's pluses & minuses w/my explanation as to my recommendation:
X800 Pro - 12 pipes, 256MB, 475/900 clock for $400
X800 XT - 16 pipes, 256MB, 525/1150 clock for $500 (Retail, you'll pay much more to get one soon)
6800 - 12 pipes, 128MB, 325/700 clock for $300
6800 GT - 16 pipes, 256MB, 350/1000 clock for $400
6800 Ultra - 16 pipes, 256MB, 400/1100 clock for $500 (Retail, you might pay more but they should become more readily available @ BBY, etc very soon))
As always, ATI has better performance w/AA & AF enabled (in general) and and nVidia has better performance in OpenGL (hands down). nVidia owns Doom3, ATI supposedly owns HL2 (if it EVER comes out).
As I've put it here (fairly un-biased IMO), the GT seems to be the best performance for the money. Most OC easily to Ultra levels if you're into that, and if not still give you awesome performance. If you want to save some money, the 6800 can be unlocked to a full 16 pipes and I would imagine in the near future you can find vanilla 6800's with 256MB, eliminating the other disadvantage of them. Clocks are lower, but for the money you can't beat it. IMO, the only ATI worth buying is the XT. The Pro gets beat by the GT in almost every instance for the same money. I also personally like nVidia's drivers better, between the interface itself, built-in OC options w/Coolbits, and game profiles so you can set specific performance/quality settings for individual games. I'm not an nVidia or ATI fanboy, as I went from a GF4 Ti4200 to a 9800 Pro and loved 'em both. In this round tho, I would say nVidia won in performance/$. ATI would win for top performance if you could find an XT for a less than astronomical price. So, to sum up: I would look at one of the nVidia's depending on your price range or bang/buck, or the XT for ultimate performance (at an ultimate price). The Ultra isn't really worth it IMO, since the GT OCs to Ultra speeds in most cases; but if you want a readily available card than the Ultra is more feasible than the XT.
Okay, enough rambling. Just remember: its only good advice if you take it
