I don't mean to copy-pasta computer specs from someone else's rig, but you might take a look at the one in this thread (more specifically the last post):
http://strawberryforum.site88.net/forum/viewtopic.php?f=11&t=265&start=10
Seems more than capable of running GTA IV, with two SLI'd GTX260s and a Phenom X2.