Yes, it's possible!
Go to the Elgato website and use the ATI TV Wonder USB 2.0
It's completley lag free since it doesn't have an mpeg 2 encoder built in, so it's super fast!
The downside is, if you want to record anything, you can't do it in super high quality, because there is no mpeg2 encoder built in. It's a double edged sword, but I'm going to be getting one to play my Gamecube on my 20" iMac!