games like BF2 will work in linux with the help of transgaming's Cedega. See the web site for a list of compatible games. Performance is a sticky issue. You may get better performance in one game and worse in another or every game could be slower. It all depends on your hardware, transgaming's level of integration and the game itself, along with it's hardware usage. The short answer would be that perfomance should be comparable with windows. There are many games built for linux, some commercial some free. See the linux gaming sticky at the top of the forum page.
It also depends what graphics card you have. If you use ATi cards, their Linux drivers as of now are pretty poor performance wise. On the other hand, nVidia drivers are excellent, and in my experience performance in Linux is much better than Windows.
As said above, if you have an ATI card, Dont bother trying.. Open GL on ATI cards Linux suck horribly. There are a few tweaks that can be done but 99% of the time you arent going to achieve the desired performance. If you have a Nv card on the other hand, you will mostlikely experience the same, if not better performance then running Window. Also, Dont buy Cedega, it is basicially just a fancy version of Wine. I use Wine under Ubuntu, and it works fine. There are a few tweaks for each game that greatly help the performance. I installed WoW on Wine with no issues, Some games require a bit of tweaking though. Also, check out Wines App Database and look up the games that you want to play, look at the ratings and read the user comments to get a good idea if it will work successfully or not.
