Why wont my Dell server use IBM ram?
I think what you're running into is a ranking issue - the 512MB sticks you picture are single-rank (1Rx8) where the 4GB ones are quad-ranked (4Rx8). Are you running a current BIOS on the 1950? I'm seeing docs that imply you need at least BIOS v2.3.1 to support quad-ranked DIMMs:
http://en.community.dell.com/techcenter/extras/f/4483/p/19417834/19983266.aspx
http://blog.mischel.com/2008/12/16/memory-upgrades/