What price range? I would defenetly look at the x800 Xl for the type of games you are playing. ATI cards defenetly have the edge against Nvidia in Half Life 2 games. Also, right now, you can get this card for 250 at monarch. This card is very competative with the 6800GT in most games, and currently kills the 6800GT in terms of half-life2 performance.
Right now though, if you are looking for a graphics card to play general games with, and not one specific game, then the 6800GT is your card. This card, priced at 300 dollars at newegg, is currently a better bang for your buck, especially if you are going to overclock it. Overclocked, this card can pass Ultra levels in terms of performance. When this card was priced at 350, it was hard to reccomend, but now priced at 300 this card is just a little better than the x800XL because it does offer Shader Model 3.0, which will enhcance the graphics quality in some next generation games, and also because it overclocks better than the x800 XL.
Also 003, the Vanilla 6800 is like 220 dollars PCI-Express. Its the AGP ones that are cheaper.
6600GT PCI-E for sale/Trade. Check the FS/FT forums on tech-forums for details.