Windows Enterprise Edition Server 2003 memory problem

I just installed Windows Enterprise Edition Server 2003, SP2 on one of my servers. I upgraded it to 6 gigs of RAM ( 6 x 1gig PC2100R EEC ) All 6 gigs were recognized in the BIOS, but the system properties only show 3.87gig.

What setting do I need to change so all 6 will show up in System Properties?