Fallout 3 is a must. If you really want to test your computer, try Crysis and Crysis Warhead (or even GTA IV, hehe). Oh, and if you're a RTS fan, buy Command & Conquer: Red Alert 3.
For your web browser, Firefox or Opera is good. For a free antivirus, get avast. For a free anti-spyware, get Ad-Aware.
Random game thought :
Why is Bionic Commando Rearmed 2 getting so much hate? We finally get a real game and they're not even satisfied... I'm starting to hate the gaming community so f****** much...