DIY 4 GHz Dual Core Gaming Rig For $720
Tom's Hardware has posted the detailed results of their recent quest to build a beefy gaming rig without a visit to the poorhouse. The trick it seems is to find a processor with 'cores designed for a much faster clock than their nominal rating at a speed of up to 4 GHz without problems.' They provide shopping lists for both a 'budget version' and a 'top flight version'.
for those who dont want to browse through n pages http://www.tomshardware.com/2006/06/12/your_diy_ga ming_rig_for_720/print.html
According to newegg the Intel Pentium D 805 2.66GHz CPU is $118.99 (free shipping). That's about $3 less than the article states.