you can build two killer systems with the budget you have. i recommend checkin out websites like www.newegg.com www.mwave.com www.zipzoomfly.com
they are all good and reputable. ive had experiences with newegg and mwave and theyve been great too me. all of my computer parts were shipped on time. the only thing i hate about buying online compared to buying local is that sometimes computer parts come dead and then you need to ship it back again. oh well this happens to all of us.
anywayz, amd leans towards gaming and Intel is geared towards more on multimedia stuff such as ed modeling and video editing. if you are building a gaming rig, either company will be a good choice. for amd stick to the socket 939 processors as they will benefit you more in the long run when it comes time ot upgrade your system.
buy brand name parts and not generic crappy parts. for example, good brands include ocz, crucial, mushkin, kingston, corsair, and patriot. antec, enermax, pcpowercooling, and ocz make very good power supplies. as for videocard, i think the 6800GT is a good videocard and if you decide to get it, get the videocard with the most bundle software.