Not all of the installed memory over 3Gb can be used:

Although Windows XP has a maximum memory limit of 4 Gigabytes, not all of the last gigabyte is available to the operating system. Some of this space is also used by video cards and i/o devices.

The amount XP will see depends on the motherboard and also on bios settings such as AGP aperture size.

Increasing the AGP aperture size increases video performance, but decreases the maximum available ram when 4 Gb is installed.