How do you tell if you OS is 32-bit or 64-bit

I want to upgrade my system and I want to use my existing XP, but only if it is 64-bit (to use 4 GB of RAM). How can I tell if my current version of XP is 32-bit or 64-bit?

I actually have two XP versions, Home and Pro, both are the 2002 versions.

I looked in "system" from the control panel and it just says XP Home, 2002 version, but does not say wether is it 32 or 64 bit.

Thanks for looking.