Well depends on the games you want to play,
Halo is a great FPS shooter for networks and costs next to nothing to buy.
I would reccomend the CE version as it means you can download new maps for network games to keep the game more exciting.
New maps can be downloaded here.
If you are after freeware games to play over a LAN then this site has quite a few, be carefull though some are trials only but the ones marked Freeware are 100% free.
With all the games even the bought ones you will need a copy on each machine so if you do buy a game you will need two copies but some older games cost not more than a few quid.