Gamecube+ATI 9700 All in Wonder
 

Has anyone ever hooked thier gamecube up to the monitor with a tv tuner card? Or any console game for that matter?

NO! but it sounds cool.

It'll be just like hooking up your VCR to it. I used to run my PS2 through a Rage 128 AIW in my PowerMac 8500 and it was awesome. The only problem i had was I couldn't get the volume up very loud for movies but that's just a PS2 problem. Now my GF uses the 8500 plus AIW with her DVD player so she can watch Sex and the City and it works great.
Do it, it's fun :)

I've hooked it up to a computer..

there are a few ways you can do this
you can run the gamecube or console through a VCR or DVD player, and just use the remote for that device to select "A/V" and you're all set.. Or i beleive there are a few vidcards with av hookups already built in.

I've got an AIW 8500DV, and it has really cool A/V inputs and ouptus. I've connected my PS2 and played it on the tv tuner. Sometimes the picture is kind of dark, but you can use the TV tuner gamma adjuster to fix it. It's fun!