In terms of the Ceton InfiniTV itself–all of them. Even the Intel Atom D510. I’m trying to get my hands on a Intel Atom D525 to test as well but unlikely. I’m honestly amazed how little resources are used.
In terms of a HTPC as a whole, the Intel DH57DD + Core i5-670 as been awesome and my ideal setup (Ref: HTPC #1). I have a full review of the Intel DH57DD + Core i5-670 just about complete that will be published here at Missing Remote. But wait for that review as there are some caveats depending on the enclosure.