In terms of fan noise, power draw, and overall performance, the GeForce 6800 GS runs neck and neck with the Radeon X800 XL. That's not bad at all for a card that starts life at the $249 mark. If history is any guide, board makers will offer discounts and "overclocked in the box" clock speeds to make the 6800 GS even more appealing than NVIDIA's stock design. In fact, I believe those cards should be available for purchase starting today.
The Radeon X800 XL, meanwhile, started life in a higher tax bracket and only recently has dropped into the $259-ish neighborhood. We'll have to see whether it will stick around and continue to drop to $249 or less over time. I wouldn't hesitate to recommend the X800 XL as an alternative to the GS, except for the fact that ATI has now embraced Shader Model 3.0 in the Radeon X1000 series. The 6800 GS has Shader Model 3.0 support, and the Radeon X800 XL doesn't. That fact may not matter now, but it could well become a source of regret down the road for someone choosing an X800 XL over a 6800 GS today. Game developers may shift their attention to SM3.0 at the expense of older cards, especially once ATI joins NVIDIA in pushing them to incorporate more SM3.0-specific features and performance optimizations.
As for the Radeon X1600 XT, let's hope ATI finds a better way to plug the gap between the $199 and $299 price points than penciling this minor leaguer into its lineup. The Radeon X1600 XT is in many ways a very forward-looking design, but it's not a good value at its current list price.
The biggest knock on the GeForce 6800 GS, in my book, comes from another source. If you're going to fork over $249 for a graphics card, you would probably do well to consider shelling out the additional $70-100 for a GeForce 7800 GT instead, if you can. The 7800 GT delivers significantly more performance, thanks in part to the tweaks NVIDIA made between GeForce generations six and seven. Similarly, if Radeon X1800 XL prices settle into the same range as the 7800 GT as supply becomes more plentiful, that card could become a very attractive option. I don't advocate reaching into a higher price bracket often, but this is one case where it may make some sense.