IMHO, you're best off using a standalone PCI capture card. If you buy a combo video card/TV tuner card (like an ATI "All In Wonder" or NVIDIA "Personal Cinema" model), you lose the video capture functionality when you upgrade the card. OTOH, if you only need video-in (for instance, if you are capturing from a VCR, or you have digital cable and need to use a STB), you can get many cards with VIVO functionality for essentially the same price as a regular video card.
I'm currently using the new Hauppauge WinTV PVR-150MCE (hardware MPEG2 encoding) in my HTPC; it's a pretty solid card for watching TV, since the CPU usage is very low and the quality is excellent. Before that I had an ATI TV Wonder VE (software encoding; works with DScaler, but not SageTV or the new Windows Media Center Edition 2005). It worked, but ate up ~80-90% of my CPU to timeshift in BeyondTV 3.