Desktop Resurrection - Need power supply for 2002 Dell desktop

Wasn't sure if this is the right category but I guess it's ultimately a question of hardware...

In case you don't want to read the whole background, my ultimate question is, Which power supply should I look to get to replace the dead one in the old unit?

I just moved into an off-campus house at college and the last homeowners left behind a lot of stuff, including what appears to be a functional Dell desktop PC. At least 10 years old, judging by the available ports on the MB and the other hardware inside. The unit is dead on its own, but I tried connecting the 20-pin part of my desktop's power supply to the old unit and the motherboard seemed to power up (light went on, CPU fan started).

The hard drive and CD-ROM drive didn't seem to power on, but I found what seemed to be a safety button (see first image) which I am guessing prevents those devices from powering on if the case is open. Can anyone confirm this for me? I want to know if anything else in this unit is dead, but it seems like everything was in great shape (not much dust, etc). I don't really want to pump more than like $50 bucks into this thing, as my goal was to use this PC as either a dedicated Linux machine or to host a web site (not expecting much traffic at all, and with FiOS Quantum 75/75 we have more than enough bandwidth).

The motherboard looks very much like the Dell OptiPlex GX260 SDT SFF Socket 478 (Image: http://attach.forum.ge/post-54-1270667615.jpg) although I cannot seem to find the exact model number without taking out the power supply (good ol' Dell plopped it right on top of the PCI connector...) The drives are NOT Serial ATA... (so I guess that makes them IDE? correct me if I'm wrong here, still learning.)

I guess what I want to know the most at this point is what is a good replacement for the power supply? From what I can read, the current one in there is a Dell PS-5161-1D (Image: http://i.ebayimg.com/00/s/MTIwMFgxNj...VEFNE/$_35.JPG) but I don't know if I should go with the same one or a different one.

Advice? Thanks in advance

Oh nice! Yeah that looks like the right one, and it even says it was designed for OptiPlex GX280, which is the same MB family. I'm just wondering, do you recommend any other product that might be better/smaller for a similar price? I'm concerned with hardware compatibility but I want to get the best one I can for cheap. Thanks for your input!

Here is the best image I got of the power supply label. I think I got the model right...?

Also why would you say it won't boot? I know it won't boot if the hard drive doesn't power up but that was the other thing I was wondering with the first image I attached to my first post... if that little button was like a sentinel for if the case is open, to cut power to those devices to prevent any malfunction.... but if those components are shot it's not worth my resources to resurrect, may as well just buy or build another cheap platform

Oh well my apologies, I though I was being more clear. When I said "the unit is dead on its own" I meant the power supply. The motherboard seems to work and other things will power on when I connect a working power supply to the MB, but not everything (i.e. disk drives). What I then intended to ask was, if someone were familiar with this model computer, if that button I have pictured is a sentinel for the case being open. Some devices (like washing machines, as an easy example) have hardware sentinels that can tell if something is open or not in position to function properly. You can't, for instance, open the top of a top-loading washing machine and watch the agitator move because the sentinel detects if the top lid is open and stops the cycle if it is opened during operation. In a similar manner, other devices may have hardware that prohibits operation until a button is depressed, a spring is expanded, etc. which signals that the device as a whole is in proper working position.

I was specifically asking if that button does this for the hard drive and CD drive, because if it does, then I will try to see if there is another problem. If this is not the case, and those disk drives are nonfunctional, then I would be considering the entire unit "not worth my resources to resurrect." As I said initially, I wanted to get this thing running for $50 or less, so the fewer things I can eliminate as causes for lack of functionality the better.