THe only thing the Xeon will offer you is multi socket support. You will need a more expensive board and most likely FB-Dimm.
If a dual core or a quad core is enough for you, you really do not need a xeon 5XXX. You could also look at the xeon 3040, essentially a e6400 with the Xeon branding(almsot the same price, dunno if Dell offer's them)
Save you money and go the e6XXX route, much cheaper!