It has a Pentium D 930 right now and I want to upgrade to an E8400. It has LGA775 and the 945P chipset so it should work but I just want to make sure, has anyone done this?
The XPS400 used 2 different motherboards the DXP051 supported the P4 and the Pentium D 8xx up to the 840 the other (can't remember the model) only supported P4's both were BTX format 945p chip sets remember on an OEM board like a Dell it a custom unit that supports the OEM's needs and the Bios more so then the chipset is the limiting factor, when you boot the pc at the Dell logo screen above the progress bar it should have the model number listed. and the Bios revision under the bar like A0x
The chipset might support a Core 2 Duo but chances are the power regulation circuitry does not and the BIOS likely doesn't either. I really wouldn't waste my time, money or effort with this.