It kind of depends on what you want to do with the card. My system is set to 720p so I bought a Radeon x1600 which is a little on the low end for HD video, but works good enough for my needs.
If you are only doing SD playback at 640x480 or 800x600 I imagine you wouldn't need something too powerful. Maybe a Nvidia FX5200, or a Radeon x300 would work ok (both under $40)
[SIZE="1"]P4 3.0Ghz, 1GB Ram, 120 + 250GB HD, ATI x1600 @ 720p
Hauppauge PVR500, (2) ATI TV Wonder 550
XP Pro SP2, HIP, FFDshow (for audio), VMR9[/SIZE]
On an SD TV set any ATI X1000 series card will do. I would not look for the cheapest but just one level higher. You get that for a very small extra and it has a lot more headroom.
I'm happy with an EVGA Nvidia 6200 for S-video out to a SDTV. This card has passive cooling for silence and, as I understand, works with the Nvidia Purevideo decoder to unload some of the processing from the CPU to the video card. Search the support (I think) forum for several threads on how to optimize Nvidia cards and the Purevideo decoder. I followed their advice and am very satisfied with the picture quality.
if you are going to output standard video resolution (to a CRT tv) go with an Nvidia 6200+ or 7300+. most are passively cooled, cheap and do the job well. If you are outputing to HD resolutions (720p, 1080I/P) go with an ATI x1300 or better. Nvidia has tearing issues on many configurations when outputing to HD resolutions. There are fewer options on passively cooled X1300's but they are out there. They also tend to be a bit more expensive than the Nvidia solutions.