I've heard it'd need great specs to be able to run sufficently because of the new graphics system. Windows XP for example uses sprites and so a 2D method to show all the graphics which is simple and easy (even for old graphics cards). The new one will use a Direct-X interface which will need better hardware when more boxes are up, effects etc.
I think that the rating of a 5 ghz PC being needed that i've heard about is over rated. As with any other tihng that comes out, hype is always bad as it makes you think less abotu it.
It'll be a new operating system. It'll look a bit better using a different interface but I don't think it'll change that much. It'll become stabler, and all, and have new programs, acccesibilities to new hardware.
I'm still holding to my guns that it won't be exactly as ground breaking as we think now, though still good of course.
Hope this helps