Hello Vladimir,
Hauptwerk can only ever see/use RAM that Windows itself allows, i.e. any RAM limit placed by the Windows edition inevitably applies to Hauptwerk too.
If you're upgrading Windows at all then my advice would be to go straight to the current version (Windows 8.1 64-bit), since in general it's supposed to perform (very very) slightly better for audio/MIDI than Windows 7, and it's also likely to have the most active support from hardware/driver makers, and application developers (like us!) and therefore the longest useful/working lifespan.
This page lists the RAM limits of the various Windows 8.1 editions:
http://www.microsoft.com/en-ie/windows/enterprise/compare.aspx