I currently have an ATI X800XL. I play Halo, World of Warcraft, Call of Duty (Hopefully CoD2 soon).
I can run WoW @ 1600x1200 easily, Halo @ 1280x1024 easily, I'm not actually sure what I run CoD @. The X800XL is pretty noisy stock, so I added a Zalman V700 cooler. Now the card is very quiet. I paid $300 for my card when it first came out, and at that time the closest competition was a 6800GT which was about $350-$400. The X800XL isn't such a great deal anymore.
My opinion is:
The lowest card to get is a 6600GT, but I would probably not bother since it is pretty similar to a 9800pro from 2 years ago.
Next, there is the X800GTO and GTO2. I'm not too familiar with these cards, but it sounds like they can be modified to run very fast. If you are comfortable modding, then these would be great choices.
After that, is the 6800GS, 6800GT, and X800XL. The 6800GT and GS are essentially the same speed, but the GS is made on a new, smaller, but higher clocked chip. This makes it less expensive for the same performance. Both cards are slightly faster in most games than the X800XL.
If you don't like modding, then the 6800GS IMO is probably one of the best deals out there, if you can find it for $220 or so. Newegg has an XFX clocked at 485 core speed for $215, and that is probably what I would buy.
However, if you can spend the money, jumping up to the 7800GT would be the next step in performance, and it is quite a step, too. Newegg has an EVGA version @ 460/1100 for $319.00
The X1800XL is overpriced and underperforming IMO, but does have a nice AF mode. Still, I don't think it is worth the extra money.
The rest of the ATI lineup are fine cards, but I just don't see them as a good value considering the current NV lineup.
Since I do not overclock, I would currently get either the 6800GS or 7800GT. If I was an overclocker, I might try an X800GTO or GTO2.
-D'oh!
EDIT: I also wanted to mention that I plan on keeping my card for at least 1 more year, maybe 2. Also, don't be worried about running games in a different resolution than the native one on an LCD. I've used a dell 2001FP @ 1280x960 many times (in Windows desktop), and while it certainly isn't as sharp as 1600x1200, it still looked very good. In a game, the difference might be noticeable, but I personally would not get bothered by it.
-D'oh!