If your computer is booting properly with just one RAM stick installed, that is an indicater that the one that is not present may be the one with a problem.
Try switching the other one into the slot and leave the one you have in now aside for a bit and try rebooting the computer.
If the computer boots up with any errors, you know you have a bad RAM stick, something is not correct with it, its either a mis-match, (incapatible module) or there is a defect.
If the computer does boot up okay with just this RAM stick, you may either have two RAM sticks that do not get along with each other, or you are not following the instructions on which slots you are supposed to be using.
I am going to go to the Dell web site and link you to some support pages for your computer.http://support.dell.com/support/edocs/syst...ps400/index.htm
This page contains links to both a user manual and a service manual.
This information may be helpful to you for diagnosing the status lights on the back of your computer.http://support.dell.com/support/edocs/syst...t.htm#wp1054146
This is the specs on your computer. http://support.dell.com/support/edocs/syst...s.htm#wp1053343
Make sure both RAM modules are what is specified in the box for MEMORY.
This is information regarding adding hardware to your computer. http://support.dell.com/support/edocs/syst...s.htm#wp1058068Please pay special attention to the page I linked you to above, please note Dimm 1 and dimm 2 are NOT next to each other, the dimm slots are dimm 1 dimm 3 dimm 2 and dimm 4
Notes I copied from that page are here:
Memory modules should be installed in pairs of matched memory size, speed, and technology. If the memory modules are not installed in matched pairs, the computer will continue to operate, but with a slight reduction in performance. See the label in the corner of the module to determine the module's capacity.
NOTE: Always install memory modules in the order indicated on the system board.
The recommended memory configurations are:
A pair of matched memory modules installed in connectors DIMM_1 and DIMM_2
A pair of matched memory modules installed in connectors DIMM_1 and DIMM_2 and another matched pair installed in connectors DIMM_3 and DIMM_4
NOTICE: Do not install ECC or buffered memory modules. Only unbuffered, non-ECC memory is supported.
If you install mixed pairs of DDR2 400-MHz (PC2-3200), DDR2 533-MHz (PC2-4300) and DDR2 667-MHz (PC2-5300) memory, the modules function at the slowest speed installed.
Be sure to install a single memory module in the DIMM_1 connector, the connector closest to the processor, before you install modules in the other connectors.
Hope this helps.
Edited by MrBruce1959, 12 August 2010 - 02:19 PM.