No the XPS 400 can only support the Pentium D series and older. I have a XPS 400 and I also looked into installing a C2D in to it.
The Fastest CPU would be The Pentium D Extreme Edition 965, If you can find one and want to pay for that.
IF you can get a XPS 420 motherboard. It will fit in the XPS 400 and support the C2D. The Dell XPS uses a BTX case, a ATX Mobo will not fit.
1Haplo