The slowest GPU i use for recording HDTV is 3870... The CPU usage is ~ < 10% average. The task is done mostly by the GPU.
I tried different GPU
3870, 4870, 4850, 4890, CF 4890
For Nvidia i still have my 8800GT, and GTX 280C.
Recording and playback HDTV movies is hardly noticesable on CPU usage.