I've ordered all the parts to build a Home Theatre PC (HTPC). It really is just a PC-based Personal Video Recorder (PVR).
The basis of it is SageTV. I'm using a SageTV bundle with the Hauppauge WinTV-PVR-350 ($219.95). This has a hardware MPEG-2 encoder and decoder. This will let the SageTV present its On-screen Display overlaid on the TV picture. Functionally, SageTV is a lot like TiVo. SageTV will support multiple WinTV cards so you can record more than one program at a time.
For a system unit, I picked up an off-lease Dell GX150, 933 MHz with 512 MB of RAM from RetroBox ($108). It comes with a 20 GB hard drive which obviously isn't enough. BestBuy had a Seagate 80 GB hard drive this week for $39.99 after rebate so that cured that.
I'm into it less than $400. Now, I'm just waiting for the PVR-350 to come off back-order.