Best HTPC setup
I've played clips that performed well, 1080p reached ~80% peak on the CPU, would be touchy without the GPU acceleration. I would get faster components if I was building now.
Playing ~15mbps 720p and 1080i ExpressVu content uses around 5-20% CPU on my system, recording isn't noticeable but it's an exact stream copy. If you're using a hardware mpeg encoder the CPU requirements are minimal.
Will the HTPC be handling all your DirecTV viewing and are you sticking with SD until they [hopefully] offer a PCI tuner? Any plans for OTA HD?