RAM's and Windows XP

Well as I discover more and more about hardware supporting software it all seems to get more complex. I discovered yesterday that although I upgraded the RAM on my lappy last year from 2gb to 4gb becasue I am running Windows XP as my OS (therefore 32bit) it cannot optimise the increased memory but can only use 3.something of it...darn no wonder it failed with renderworks, if only I'd known I could have simply upped the OS and the RAM instead of buying the Mac i5..........ok well thankfully I didn't know so got the joyous i5 as compensation for my own lack of research!