Hmm not much you can play with that computer. Source games like counter strike source and Team fortress 2 might work on low settings but thats about it. Try battlefield heroes, its a free 2 play online game. Other games include call of duty 4 which i myself finished on a E5400 (about the same performace as your athlon) and an integrated intel graphics card. You can get a used graphics card for around 20 bucks that will let you play most games but not the latest ones. That processor is okay but don't expect too much from it
Max Payne, Duke-Nukem 3D, Doom 1-2, Blood 1, maybe Half-Life 2, Half-Life 1, Quake Live (its free), deus ex, Call of duty 1, system shock 2, Wolfenstein 3D, almost all Command and Conquers, Titan Quest?, Halo, Terraria, Counter Strike 2D (its also free), tyrian 2000, Fallout 1, 2. Take also a look at this website: GOG.com
Your PC should be able to handle any game on GOG as well as many more recent indie games. However, it definitely may be worth investing in a cheap graphics card. I'm guessing your PSU is around 300 Watts? If so, you could get a Nvidia GT440 or a Radeon HD 5670 which will allow you to play more recent games. I have a GT440 in my older PC which had similar processor / memory to yours and was able to run Skyrim with max settings at 1920 x 1080 (I think I turned off AA which you don't really need at that res).
Hey friend you can play the games like NFS most wanted ,Warcraft ,Age of empires ,Freedom fighters ,Prince of Persia -warrior within and Brother of arms also this games will be bit old but good to play intrusting game which will relax you after a long time work