Some people have claimed Adobe has let them upgrade crossing platforms; I would contact Adobe directly before buying the upgrade version though.
She'd have to have Photoshop 7, CS, or CS2 to be eligible (assuming they allow it).
It's also quite possible that Photoshop Elements will do everything your wife wants, and for a much lower price. Since she's switching to Mac, I'd strongly suggest waiting until the next version is out (supposed to be early 2008) - the Mac version currently lags significantly behind the Windows version.
Edit: The upgrade eligibility page I linked to, above, explicitly states that it has to be the same platform. So I guess the folks that've claimed cross-platform upgrade eligibility are blowing smoke. So check out Elements!