Yes, i think the limit of XP64 is 128Gb.

There are some ways around the 4Gb limit on a 32bit system but it involves how the memory handling is programmed and on XP its not done in the way that it can handle any more.

If you look at this list you will see that some of the 32 bit systems can make us of memory above 4Gb.

http://msdn2.microsoft.com/en-us/library/aa366778.aspx