Hauppauge actually have a PCIe version of the HDPVR now. At least I recall reading about it a little while back.

If you're going to capture HD video, you'll also need a lot of disk space - at least until you transcode your captures. I'd probably go for some other barebones system rather than using a nettop for the server. If a client system is desired, then that's suitable for such a system. Something that you can cram at least 2TB into.