My gaming is mostly limited to quake3 and it's mods. Right now I play True Combat mostly. Using winex I've installed Diablo 2, Black and White, War Craft 3.
Unlike Quake3 these games run at a performance penalty compared to Windows due to the Wine emulation, but with my OC'd 1700+, 256 megs of RAM, and Geforce2 GTS-v these games run just fine at high quality settings, even with a whole Gnome desktop churning away at the background. Diablo 2 does slow down with lots of transparences in caves and stuff, but it's not usually realy that noticable.
BF1942 is purported to work, but since it's a full-fledged 3-d combat stuff you'd probably want good hardware to run it on in Linux.
To install WineX on Linux, you pay the 30 bucks or whatever it costs at transgamer's website and download the correct Point2Point binary package for your OS. You install the package and then run it. Then you use the Point2Point stuff to log in and install Winex for your user. There are good directions about it.
Other games like Quake3 that run natively in Linux the performance is comparable with windows. Sometimes a bit faster, sometimes a bit slower. Pretty much similar performance on similar hardware.
There are also some lower budget places like
www.garagegames.com that make games for linux and are quite fun.
If your looking for linux to be a good gaming platform, it is not. If your a hardcore type gamer it probably be a mistake to drop windows, If you play games 50% of the time or something like that, it'd probably be a good idea to have a win98 on a partition and then just dual boot then use Linux for everything else.
It's a pitty too. Linux would make a very good gaming platform.
edit:
linux games website Linux games tome