Depends what kind of games you like. A 600MHz Celeron should be sufficient even for Quake 3, though you'd probably want some more memory.
Frozen-bubble, as macemoneta suggested, is good fun (otherwise know as bust-a-move on consoles).
Have a look at www.linuxgames.com, www.liberatedgames.com
and search for games on www.sourceforge.net.
google will probably turn a few up too.
If you like the old point-and-click adventures (like monkey-island), liberatedgames has a link to the classic Beaneath A Still Sky, released as freeware a while back by the developers, Revolution Software (www.revolution.co.uk
). This is an extremely funny game. You need scummvm, which you can get from www.scummvm.org.