Dont even buy a dell, as they are overpriced with their XPS series.
Dont ever buy an alienware, as they are overpriced and rip you off.
Buy from www.ibuyers.com www.cyberpowers.com
i know that www.monarchcomputers.com
So you have about 1300 dollars to spends USD.
MSI K8N NEO4-F
eVGA 7800GTX or the X1800XT as you play half life 2 most of the time and half life is ATI based. Just like Doom is nVidia based.
Corsair valueselect 1GB
Seagate barracuda, however big you want it, sata150 hdd
Antec truepower 550W power supply
Case is more of a personal perference, Thermaltake Soprano is one of my favorites.