32-bit can address 3gb-3.5gb depending on your graphics. If you've got crappy graphics it'll only use 3gb, but if you have a good graphics card (scores at least 4.5) then it'll let ya use that other .5gb. SP2 of Vista and all versions of Windows 7 will show up to 4gb, but it's unclear whether they're just reporting the 4gb or actually using it.
64 bit can address up to 192gb, but Vista and 7 have a limiter on them. The home basic versions can only see 8gb, while home premium can see 16gb. Only Ultimate, Enterprise, and Business can address the full 192mb. Starter versions of Vista/7 can only see 2gb in BOTH the 32bit AND 64 bit versions. I don't know why someone would load a 64bit OS that could only address 2gb though. :unsure:
