32bit OS - WinXP,WinVista,Win7, WinServer (whether legit or not) = 4GB maximum memory address, 3GB-3.2GB maximum memory 'seen' by the OS because the 3-4GB area is for system resources mapping (you may even see only 2.98GB in windows if you have a 1GB-2GB video card since malaki kumain ng memory map ang video card)
http://en.wikipedia.org/wiki/3_GB_barrier64bit OS - WinXP, WinVista, Win7, WinServer (whether legit or not) = 128GB (WinXP/Vista) aximum memory address, 192GB (Win7) maximum memory address, 1TB (Windows Server 2003) maximum memory address, 2TB (Windows Server 2008) maximum memory address.
If you are using a 64 bit OS, your only barrier to memory 'bliss' is hardware limitation of the # of memory slots, memory capacity of the actual module, and memory capacity of the chipset (northbridge) or of the memory controller of the processor (for AMD / Intel Core iX-YYYY).