Most quality video cards will allow you to do this. As long as they have a monitor out, or even firewire out (to a Deck with outs to a monitor). If you can view your footage in your video editor on an external monitor, you should be able to find a way to do it in After Effects. I personally had to call Canopus directly and get a special plugin to add to the After Effects folder to make it work for me. There is an option you have to make sure is checked in the preferences to enable the monitor as well. I’d tell you exactly what it is but I’m rendering right now. 🙂