Need some help on AV, specifically, using my monitor as a TV

[Andrew Thoreson]

This is kinda hard to sum up neatly, so I'm sorry the title is somewhat
confusing. Basically, I'm wondering if it would be possible to hook up
devices that would normally go to a TV to my computer and then watch
them on that. 

For example, hook up my Wii, and then have the video from that show up
in a window (or fullscreen), so that I can use my monitor instead of my
TV, and I can take screenshots of it. If this is possible, what kind of
hardware would I need to take a video-in signal, and what programs would
be able to have it stay in a window and/or run it in fullscreen? Also,
potentially, record video. 

I've seen some distros that are made to function as DVRs/media devices,
but I'd prefer to keep my Ubuntu install that I have now. I've seen some
talk of setting up TV tuners, but those seem to just receive broadcast
signals, which isn't really what I'm looking for.