I have a hard time recommending the P4 build if you need to buy more hardware to complete the system (though a Gb-NIC & case won't go old and can be used in a later system). A P4 might be faster than an Atom but the Atom-ION system you linked should do HD without too much trouble, it is silent and will draw very little power. Then if you later on decide to get a beefy core you can easily use the Atom system as an MD without it being in the way. It is a bit larger than the Acer Revo but you do have a DVD player.
Either way will work fine for a core although if you go for the Atom don't forget to buy a PCI-E NIC as well.