Basically, going from isometric to full-3D is a huge risk with a franchise like Fallout, but if there is one developer that can do it, it is Bethesda.
Despite the huge system reqs of Oblivion (which I now play in complete modded graphical glory) this game should stil be a feast for the eyes even on minimum settings, which I expect to be a 6800 at least, and at least a basic AMD dual core. If you don't have that kind of machine even now, then don't complain. You have no right to moan about PC requirements. If you do, then buy the console version and be happy with the watered down dumb version you end up with.
It may sound harsh, but fuck it. PC users get the best versions of games, no matter how late. Deal with it.