Phil,
One other thought, check the version on you BIOS and then check the release notes on the most current BIOS offered by Dell to make sure that one of the intermediary revisions didn't change to support large amounts of RAM. I upgraded a Dell workstation a few years back that required a BIOS upgrade in order to take 1 GB. If you stick in the new RAM and it won't boot, a new BIOS might cure it. Dell's BIOS installs are painless, IIRC.

-Zeke