ASUS Radeon 9800XT video capturing

[lightweave]

I have an ASUS 9800XT gfx card which has also video in, but according to the readme from the driver it says that videoin is not supported. Is there some way around it so I can videocapture?
If not then what would be the cheapest solution, that works under linux? USB camera, TV card, etc.?

What I want to do with it is, to connect a camera to my Linux box and then do a screencapture every 3 seconds as a security record. I don't need real video captruing, it would suffice if I can capture single images from the commandline.

Any hints how I can do this?

Thanks!