Find a mobo that is known-good for Linux/Ubuntu using an nVidia chipset (6200-7300 for graphics will make it the easiest compatibility-wise, you certainly don't need anything more than that anyway!)
Decent speed CPU, plus 1GB or more RAM. Take a look in the wiki for PVR cards, there are several that are specifically supported, but often you will find that if it works with Linux then you are most of the way there anyway.
Use the DVD (not CD) install, it will blank the whole drive, so take precautions! Probably best to target i386. The release version is 0710 (more specifically 0710RC2).
PLEASE read both the FAQ and Installation Instructions, and it would be a good idea to look at the hardware stuff in the wiki and forums as Zaerc pointed out.
2 NICs are necessary - you need to set up a LAN for LMCE, search the wiki and forums with "2 NIC" if you need an explanation as to why. And do not mount or symlink anything manually, if it doesn't find your shares/media then you have some other problem. Manual symlinks and mounting will break the system, it does all that automatically as long as everything is working fine.
btw - if all you are looking for is a PVR, then you might find this a "sledgehammer"! LMCE is massively more than that, and consequently a greater investment in setting it up....