The best site I can give you is
http://www.newegg.com
I built my computer entirely from parts from this store. $2100 got me a 3.2GHz P4, 1GB memory, 200GB HDD, Nvidia Geforce6800 graphics card, Creative Audigy2 ZS soundcard, Creative 5.1 THX 500watt speakers, and a extremely nice Sony 17" monitor with DVI-output.