The 6200 cards tend to be pretty low end but they should be a decent match for that system. Even a 5200/5500 should work though, it's not like you'll be able to play HD video on it anyway.
BTW, the tuner cards based on the new ATi 650 chip should be coming out any day now. It's supposed to deliver the best analog video of any chipset to date according to anandtech. It also supports digital and DVB-T... though your CPU/graphics may be too slow to display non SD content. http://www.anandtech.com/multimedia/showdoc.aspx?i=2778
Also, the 6200 supports shader model 3 which some games will like where the 5200/5500 don't. If you can, get a card with at least a 128 bit buss. You are less likely to have playback problems.