There was a PC vs PS3 vs 360 graphics comparison on gamespot recently:
LINK
By the looks of it PS3 is the least powerful machine of the three. If you look at the trees or distant details on Fallout 3 they're really bad. 360 and PC are about the same apart from weird gamma settings (and the tress are also a bit dodgy on 360). Of course graphics aren't everything but it's quite interesting none-the-less.