Balanced Technology Extended (BTX) System is a specification and guide for the designers of motherboards, cases, and power supplies (PSU) to ensure their products work together.
For further information...
Balanced Technology Extended (BTX) System Design Guide v1.0
http://www.formfactors.org/devlist.asp?FFID=12&CatID=1
Dell uses a propriety PSU that has different wiring/connector arrangements than the standard ATX PSU and they are not interchangeable.
Simply, you will have to use a Dell PSU.
For further information...
Balanced Technology Extended (BTX) System Design Guide v1.0
http://www.formfactors.org/devlist.asp?FFID=12&CatID=1
Dell uses a propriety PSU that has different wiring/connector arrangements than the standard ATX PSU and they are not interchangeable.
Simply, you will have to use a Dell PSU.