About all I can suggest is that you look at the motherboard and see if you can find out who made it and what model it is then go look it up on the manufacturer's website.
As and aside: Compaq have always made non-standard systems and have been very reluctant to give out information other than "Our upgrade is $xxxx.xx". Next time you buy a computer, get one from a local computer store (or build one yourself) using standard, brand identified, parts. You'll be a whole lot happier when upgrade time rolls around.
<b>(</b>It ain't better if it don't work.<b>)</b>