32 bit Windows’ support only 4 GB of RAM except 32 bit server editions which can read more than 4 GB. In case of Windows Vista, Windows 7 (and also Windows 8) 32 bit versions (x86), up to 3.50 GB of RAM is usable even if you have installed more than 4 GB of RAM. This is due to PAE (physical address extension) limitation of operating system. By enabling PAE, one can access more than 4 GB RAM on 32 bit version of Windows as well if processor supports x64 architecture.
32 bit Server editions of Windows can access more than 4 GB of RAM because they have pre enabled PAE. In Past, Windows 2000 Server could not read more than 4 GB of RAM but Windows 2000 Advanced Server and Windows 2000 DataCentre Server editions were designed to take advantage of more than 4 GB of RAM.
If you want to get real benefit of more RAM, buy 64 bit edition of Windows. Before buying, make sure your processor can support 64 bit operating system.