The way the w-xp memory manager works for 32 bit machine the 4GB RAM is equally allocated between SU and the kernel so only 2GB is allocated for SU. If your machine is large block address aware then you can enable the 4GB switch ( via the boot init and the allocation will be changed to 3 GB SU and 1GB kernel but then some slow down occurs. For a 64 bit machine SU can use the total 4 GB if you have additional RAM.