Processor - If it's a hybrid which you intend on turning into a standalone core at a later date with lots of MDs, get a half decent dual core. Don't go mad; LMCE needs only modest hardware. If it's a hybrid that you intend on turning into an MD at a later date, a single core processor will probably do you. Again, it does not need to be powerful, if it is specifically for LMCE don't spend too much money. The crappy computer in my workshop must be 7 or 8 years old, and it works fine (as an MD!).
Motherboard - If it's a hybrid/core, might want to go fullATX for the extra PCI slots (remember you need a second NIC when it comes to booting MDs). If it's going to turned into an MD, microATX should be fine. Or, consider getting a miniITX board, which has processor built in (just make sure you get an "ion" one - this is nVidia). Obviously you will be very limited on PCI slots, but miniITX boards make fantastic MDs in my opinion.