Just remember 32bit OS's have a usable RAM limit of 4GB (with the exception of Windows7 Starter edition which is 2GB)
And all the different flavors of windows 7 (64bit) also have different RAM limits.
Windows 7 Home Basic (64bit) = 8GB
Windows 7 Home Premum (64bit) = 16GB
Windows 7 Pro and up (64bit) = 192GB