"The power supply that came with your Dell computer, manufactured since September of 1998, may look like a standard ATX power supply. A standard ATX power supply will even physically fit into the Dell computer and the electrical components will look like they line up, however, if you don't use a proprietary Dell power supply or a Dell to ATX Power Supply Converter with your standard ATX power supply, your computer will not work and damage may occur to the motherboard and/or the standard power supply.Your Dell Power Supply May Be Proprietary
Like many other pre-fabricated computer manufacturers, Dell has chosen to go with many proprietary computer components instead of using industry standard computer parts. When it is time for replacement or upgrade of these components, they have to be purchased from Dell. In Dell's case, one of the proprietary parts is one you might not expect: your Dell power supply may look like a standard ATX power supply, but in fact cannot be replaced with a standard ATX power supply. The reason you need a proprietary Dell power supply is because Dell has been using specially modified ATX motherboards with non-standard pinout of the motherboard power connectors. This Dell to ATX adapter converts between the non-standard pinout to the standard ATX pinout and turns a standard ATX power supply into the equivalent of a Dell power supply.
Adapter To Turn Standard ATX Power Supply Into A proprietary Dell Power Supply
If you own a proprietary Dell computer and wish to use one of our quiet power supplies (all of which are standard ATX power supplies), you can simply add this converter to the quiet power supply and have a quiet Dell setup.
Pentium 4 Dell Models that do not require a power supply adapter:
Dell Dimension 4300, 4400, 4700, 8200, 8300, OptiPlex GX 400
Pentium 4 Dell Models that are not compatible with this power supply adapter:
Dell Dimension 8100
Notes: If you are unsure if your Dell requires one of our Dell Power Supply Adapters, please take a close look at the Dell motherboard pictured at the top of this page and see if your motherboard has the extra connector pictured on the left. Also Some Dell machines chassis may need to have material removed to make room for a standard power supplies ON/OFF switch. http://www.endpcnoise.com/cgi-bin/e/std/sk...lconverter.html
I found the above in a Google search for "power supply for dell computer"
It states that Dells use a proprietary setup and the link on the page offers a converter - but warns of possible fit problems.
You have run into one of the problems of buying a computer that uses proprietary parts instead of industry standard ones.
I am not saying you can't accomplish what you are trying to do, but research it fully and exercise caution.