I just found this site and it goes on and on about Win XP Pro 32-bit with a 4gig memory cap although it may not all be used..
"This 4GB space is evenly divided into two parts, with 2GB dedicated for kernel usage, and 2GB left for application usage. Each application gets its own 2GB, but all applications have to share the same 2GB kernel space."
"basically in any 32 bit windows, you can't run a program that uses more memory than 2gb, unless you use /3gb or PAE with a PAE-aware application.
So having more than 2gb won't necessarily help for games or any other single application, and normal versions of XP/vista 32 won't use more than 4gb of ram."
" Vista uses more ram than XP to run the basic OS. It also uses any free ram for system caches and pre-fetch, which is the reason lots of panicked early adopters complained about having no free ram. These caches don't do anything bad though, they get dropped as soon as any application actually needs that ram. "