Well,there is some things to take in account. First: system specs as mentioned above. If you are going to play newer games, you'll need a damn good machine.
Then you need to find out what kind of games you will be interested in. I would suggest trying demos and whatnot, to try and avoid buying games that looked good on the box but in fact actually sucked. Lots of money wasted here, believe me. Having high speed internet helps, if you want to play multiplayer games and download the demos. Im a first person shooter gamer so high speed for M.P. is imperative.
ANd depending on your age/marital status, will mom/girlfriend/wife, let you spend the time on the computer playing games? The women tend to get pretty mad when you when you dont spend that "QUALITY" time with them.
Some games i have:
Battlefield series
SPlinter Cell (waiting for Pandora Tom.)
UT Goty
UT2K4
Far Cry
StarWars JK2 & JKJA
Soldier of fortune series
Medal of Honor AA and Exp.packs
Many Many more, too many too mention.
Places i get my demos from:
http://www.gamershell.com/download_demo.shtml
www.fileplanet.com
www.gamespot.com
Confuscious say: He who go to bed with ithcy ass, wake up with stinkyfinger