thanks very much for the response!
I wasn't laptop memory (I'm not THAT dense
but I do appreciate the suggestion.
After much searching, I found out the problem.
My system (a Dell Precision 490) is a workstation, and only takes fully buffered EEC memory, which I guess seems to have different slots, even though it is DDR2.
Now off to take back this ram and start hunting for the stuff I need.