There are less commercial games available for GNU/Linux then for Windows. (this is slightly better on the mac compared to windows)
If you want to play modern 3d games then you will need a proper videocard to support this. Nvidia supports its videocards in GNU/Linux reasonably well (although they don’t open source their drivers but that’s a different story). If you need a new videocard then don’t buy ATI/AMD videocards.
Ubuntu Gamers Arena a gaming website dedicated to gaming on Ubuntu.
Here’s a list of native games which run (generally) well in Ubuntu. Native games are intended to run well on GNU/Linux. There are a couple of games such as the ones by id (the maker of quake and doom 3) that run native on GNU/Linux, windows and mac.
A few games that work very well are:
- Diablo II
- Warcraft III
- Max Payne and Max Payne 2
- Medal of Honour: Allied Assault
- Grand Theft Auto III & Vice City
- Battlefield 1942
- Guild Wars