jimmybob_90, this is untrue.
If you plug a non dell power supply into a dell computer it will not blow up your computer, nor will it blow up your computer if you plug a DELL power supply into a non-dell computer.
I know from experience, having done it myself.
Power is power, the only thing that matters is whether it is enough or too little and whether or not the power is delivered consistently.
If your requirements are 200w and you put in a 1200w it won't fry anything in my experience.
That being said, DELL cases have the hole for the power cord in the wrong place in the back of the computer.
If you slide a non-dell power supply into the case you will find the hole for the power cord to be completely covered up by the metal case (it won't fully slide in because of this.
If you have metal cutters and you are willing to endure a non pretty looking case you can just cut out the portion of metal that covers up the power cord connection and the power supply will go all the way in.
The DELL cases have some metal tabs that go in screw holes instead of screws in some places, but the holes for screws are in the same places so these tabs will just slide right into the screw holes for any regular PSU.
Aside from the fact that you will have bent metal hanging out of the back of your computer and the associated ugly factor there isn't anything different performance wise by doing any of this.
The case is structurally sound enough to be unaffected by removal of metal in this area as well.
That being said, low end cases are really cheap. You can get low end Cooler Master cases for like $30, an AMD Phenom 2 x4 840 processor with motherboard for like $60. 2x 4GB sticks go for like $40 if your ram isn't the same type as the old board.
If your computer is worse than those specs, it may be better to just spend the $130 on a whole new setup. The rest of the parts you could just transfer over like video card, power source, hard drives, CD drives, etc. Dell Monitors work just fine with non-dell computer cores if you have the right drivers, etc.
Something to consider in comparison to the cost of used DELL motherboards on ebay.
Going that route, you would be free of DELL's ultra expensive replacement part grip.
Also note, if you do end up getting a DELL motherboard off ebay and it isn't the same type as you had before, DELL will tell you that you have to use DELL RAM and stuff in it, you don't as long as you make sure that you get RAM with exactly the right specifications. Other company's RAM will cost like 1/3 as much and work just as well, but you have to download the DELL manual for your new motherboard and see the exact RAM specifications from it.
Putting the wrong RAM in a slot can jack a lot of things up, but everything is pretty straightforward if you do your research first. If the one you get on ebay has enough RAM already for your tastes, don't worry about this obviously.
- Edit - It is important to make sure the power source has the right cables for connecting to a new motherboard if you get one, a 20 + 4 setup will work for both 20 and 24, but a straight 24 will not work in a 20 unless you risk physically cutting the connector so that it becomes 20 + 4 instead of 24. If you did that, though, it would work if done properly. Just wanted to throw that out there.