Having built two HTPC boxes with lesser hardware, I'm not sure what the problem is you are having. My wife's MC box is a 2 GHz dual core with an ATI video controller and can record four HD shows without breaking a sweat. I also had a HTPC I built with a 1.8 GHz Atom dual-core and ION GPU that could handle six HD recording with some effort, but it worked.
What I didn't have were the XBOX 360 media extenders. Maybe they need a lot of juice. I now do have one XBOX 360 media extender, but it's pulling from a more powerful i5 powered HTPC with six HD tuners.