Considering I don't have Windows 7 I'd be hard pressed to give you an answer. I have Vista *hides in shame*
But to answer anyway my Windows installation takes 20 gigs, twice as much as Linux. But: for 0.083$/gig (64MB, 7200 RPM, SATA III: 1TB HD for 86$), I think the 83 cents (for 10 gigs more for Windows) is worth access to any game or application I want.
Is AO worth enough to you to pay 83 cents more in HD space to install Windows?