Dimension C521 - RAM

I have upgraded my RAM to 4GB. Each of my four 1GB sticks contains 996MB of RAM. I have checked each stick indivually and all four work fine. When I install ANY 3 of the four sticks I have 2.89 GB of available RAM. Then, I add a fourth stick and my RAM only has 3.37 GB of RAM. Where does the 603Mb of RAM go? As far as I know all my drivers are up to date and I have the latest BOIS installed. I've ran Dell Diagnostics and all the hardware is fine. My system works perfectly but my Windows XP Media Center edition Task Manager lists my RAM as 3.37 GB. Even my BIOS show that I have four 1024MB sticks of RAM. It almost sounds like there is a bad motherboard connection to one IC on each of the four sticks. Does anyone know about this type of problem and/or a solution?