I currently host PMS on a Q6600 quad-core server. I've also tested it with a heavily overclocked E5200. Both seem to work flawlessly transcoding everything including 1080P MKV content.
However, I have some friends with older single-core PCs that definitely wouldn't be up to the task today. But from the reading of recent threads it appears a variety of improvements are planned for PMS to reduce the need for full transcoding of MKV in addition to other format supporting improvements. Am I correct in thinking that this will reduce the requirement for heavy duty CPU to play back MKV and other HD content? If so, this will scale PMS's usefulness to a lot of users that don't have the resources to purchase a new PC to host PMS to serve their PS3 HD content.