"OPM881"
apples have stuff just as good, but the reason why they generally have lower processor power (down around 1.8-2.2ghz) is cause they are able to use their resources alot better then windows. my mums old apple laptop had 256mbs of ram, and a 1.4?ghz processor and was able to outdo my toshiba laptop with 2.4ghz processor and 512mb of ram. but seeing as ya wanting to stick with pc(dont blame ya, more compatability and upgradability) get a 3.2ghz dual core(that way you dont need to upgrade in the next few years), 2gig of ram, 150-250 gig hdd, and atleast a 256mb nvidia or ati card as that way you dont need to upgrade too badly in the next few years.
Thats the UNIX in OSX (hence why the X and not 10). It is more efficient, but if you end up running all of your games in a virtual computer, you're losing all of that. And even with another bootable OS (via BootCamp) you'll take the hit from the lower clock speed.
Mac's are the best for gaming. They'll work, especially if you like Mac, are familiar with the OS and enjoy it's other advantages. If you want the best, go for something thats built for Vista.