32 bit Windows has a feature called Physical Address Extension that (theoretically at least) allows the operating system to utilize up to 64 GB of RAM. I've never tried it (it has to be specially enabled) so I can't say how well it works. Info here:

http://msdn.microsoft.com/en-us/library/aa366796(VS.85).aspx

That kind of thing was a kludge from the dark ages of computing. I don't think there's any reason to use that stuff now unless you're a company with custom written software that depends on it. Just get 64 bit Vista.