I actually think that windows 8 will use less system resources than 7.
They are doing it for consolidation; apple is doing it as well with IOS.
So that one OS is consistent through all their devices; that brings greater compatibility between tablets/phones/tablets and makes it easier on devs because they just have to develop for one platform. Imagine being able to play freelancer on your phone/tablet/lappy; that's what they are shooting for.
As someone who owns a mac as well I think apple doesn't get enough **** for pulling the "We finally brought 64x support to osx 5 years after the party is over, fork over $100 and its yours" move. Windows 8 is all good with me, i'm keeping an open mind.
P.S: if you still have XP I feel really sorry for you. Unless your on 10yr old hardware w7 is better in every single way. Lacking dx10 and dx11 support your not going to get the massive performance improvements for newer graphics cards. Even if you have hardware that was released with XP the overall results between the two are in a dead heat. If you have a new computer; windows 7 all the way!