Because 32bit Windows can only use ~3Gb of ram. If you want to have full access to 4Gb or over then you need a 64-bit Windows version.
Take a look at this and it will answer your question. http://msdn.microsoft.com/en-us/library/aa366778.aspx
Thanks for replying Woofa.
I eventually figured this one out last week (always lots to learne in IT). I installed Vista Ultimate x64 over the weekend - Problem solved, thanks!