Xp Pro to 64 bit xp or vista

[abbabb]

32bit OS' can use 4GB, up to reserved address space for hardware.

All expansion hardware (Video cards, sound cards, network cards etc etc) are memory mapped, so they use up address space that would otherwise be physical RAM.

The average user with 4GB of RAM will wind up with something like 3.6GB of usable RAM. More with less expansion hardware, less with more hardware.

 

 

Win 32 bit can manage total of 2^32 = 4G of addresses which be shared to memory and other hardware locations

[jrkr]

Have never tried vista myself but drivers can be a real pain with 64bit XP. It's normally not too bad for mainstream stuff like motherboards and video card but the small stuff like usb devices (eg. usb to serial adapter or usb IR adapter).

Also there are some programs that do not work with 64bit XP and not just small freeware but some major commercial software which can be fairly expensive if you have to go out and buy a 64bit version. You could try setting up a dual boot with 32 and 64 bit OS if you have problems.