I said give up and sell your computer for a new part
In my experience, Dell, Compaq, and Gateway are HORRIBLE
Upgrade Machine.
In fact, it is fair to say that these machine are not Upgradable, much like the Apple iMac.
The Dell machine tend to use a Proprietary ATX Powersupply, that is neither TRUE ATX nor ture AT - it is a weird cross in between. I try replacing the Dell power supply with a standard ATX power supply to save $$$, but that just don't work.
My past experience with Compaq is that they tend to HIDE their Computer BIO on the HD itself. So a HD failure can wipe out the SystemBIO as well.
Search around the newsgroup. You will no doubt find many tale of people trying to upgrade their Dells an Gateway only to find it really isn't all that compatible with the Open-Standard
