Unless you’re doing some kind of post-processing on recordings (removing commercials, or transcoding for mobile devices), recording is mostly I/O bound. That is, it is neither CPU or GPU intensive. Your GPU gets used during playback of content. While recording content the GPU is likely idle, and the CPU is pretty much waiting to shuffle data from one I/O path to the other (tuner card/network -> hard disk).
My main HTPC uses a i3-2100T and I have a couple of other HTPCs that are running a Pentium G620T, and all systems work great.
