Building a Home Theater PC (HTPC) is a great way to get exactly what you want in terms of PVR features and functionality. The problem for first time builders is there are so many variables. There are loads of video capture card options, there are at least a dozen options for PVR software to run it, and then there's the most basic question of what processor and case to power it with. Prices can vary widely depending on what options you chose but expect a baseline of $500 or so and a top-end in the many thousands of dollars.
Engadget has a great step-by-step guide to building a HTPC that is powerful, capable, and costs about $1,000 to build, which is a good target price to shoot for.
They end up with a 3Ghz Pentium 4 box with tons of RAM and ample hard disc space, but I'm kind of surprised they didn't opt for a HDTV capture card (they mention it would add about $100 to the cost). It sounds like the processor is fast enough to record and playback HD content, which should be a goal of any HTPC built in 2006 (even out in the sticks I find I can get 2 or 3 HD channels over the air).
When HDTV capture cards that support the CableCARD standard come out, I just might be building something like this to record Comcast HD content at home, so I can chuck my Motorola 6412 receiver into the trash where it belongs.