Since you have the desktop style case, you have a problem. Only a Dell PSU will work due to the size of the PSU used in this case design.
Confirmation can be found here: http://en.community.dell.com/support-forums/desktop/f/3514/t/19438866
You may also run into a problem trying to mount a non-Dell PSU. The heatsink design used by Dell with these older systems uses a non-standard fastener layout so you can't typically mount a standard heatsink to the motherboard (the holes won't line up).
Honestly, you need to consider saving your money and building a standard rig.