if u are using windows xp sp2 with 4 g.b of ram i will not show u more then 2.5 - 3 g.b be coz 32 bit soft like windows don't make more then 3 g.b of ram available to you.also u can check more about your motherboard as if it can supports 64bit windows then go for it.It i will show u 4 g.b. also don't mess up with the bios or cheat your o/s as it may harm ur pc performance.so y not try 64bit but be sure about the drivers u have for 64 bit o/s.