Do you want another monitor in clone (duplicate) mode that show exactly what the user sees, or just a monitor that shows the preview window? For the former, I’d say any decent monitor with the same resolution as the monitors you already have should be fine. To hook it up, either get a splitter box that plugs into the computer in place of the current monitor, into which both displays connect (one input, two outputs – duplicating the display – ~$50 I think) or, if you have an extra display port, just plug the second monitor into the computer and tell the OS what to do. If the latter (just the preview/composition viewer), you can set up both Premiere and After Effects to send video out through Fire-wire. At my school we use a Sony DV tape deck (~$1000 a couple of years ago) as a Fire-wire device and take the TV out and plug it into a standard TV screen. This is, of course, a low-end SD-only solution, but the preview window in either After Effects or Premiere isn’t that high quality unless the time line is rendered (PP) or pre-rendered (AE). For a client watching the editor/compositor working, it won’t usually be that High Quality. If you do want HD output from Premiere Pro or After Effects, you’d need a card of device meant for this, at which point I will bow out and let someone more experienced than I am take over.
Bruce Wainer