Well, the 4ghz requirements for Killer Instinct 1/2, Cruis'N World/USA, War Gods, and Primal Rage have been surpassed by the 7.6ghz requirements for these games.
MAME is the ultimate benchmark for a cpu, I wish people would realize that. You can unthrottle a game(run it as fast as your cpu can with an FPS counter) like Street Fighter: The Movie which requires a 1.3ghz CPU and compare fps numbers with friends, or whatever. MAME gets absolutely zero improvements from high-powered video cards or dual cpu's though. One cpu better be able to cough up 4+ ghz.