Dell has their mobos made for them to spec by the likes of IBM, Intel, and Asus...
If you are planning on replacing the mobo in your Dell as part of an upgrade, let me save you some time and say that you should just start from scratch...get a new case, psu, mobo, proc, and memory...the most you can really salvage from a Dell prebuilt machine is really the hard drive, floppys, memory/cpu (depending), and the CD/DVD ROMs...