Unplug the tower from the power, pull the bios battery out and press the power button to clear any stored energy in the PSU. Then put back the bios battery and see if you can get it to boot with the memory.
Anyways, I see what you're saying about the manual. It does say it supports single or dual memory of ddr2 667mhz and 800mhz types.. It's really basic info and doesn't go into details much like QVL (Qualified Vendor List) would. Some memory just will not work for whatever architectural reasons. That and Dell in the past was and still is proprietary in both design of cases and hardware, motherboards, PSU. By the sound of the error it might just be that the bios is outdated.
Support webpage for the machine...Here Be sure to double check that your motherboard, system model & serial numbers match. There is always a possibility of different hardware in a machine than it originally had if you bought it used and not new.
Edited by technonymous, 09 November 2016 - 02:43 PM.