Last Christmas my son built me a high power gaming computer (my hobby). Last week we updated it from 2 gig of memory to 4. The system info showed 4 gig but only 3 gig were being used according to other system information tools. We took the computer into the Geek squad and learned that the basic XP Home Edition addresses a maximum of 3 gigs of memory. For most computers, however, you still have to put four gig in because they will not function on odd numbers of sticks (i.e., 3 or 5 sticks).
Thought this info might be helpful to others.