What resolution do you run on your gaming machine? I've seen video capture cards and hdtv tuner cards at stores for around 100 bucks that advertise capture at 1080p (1920x1200) so worst case you can always spring for one of those if your wifes box can't keep up with the stream at a decent framerate. You would also need a video distribution multiplier ($$$) to duplicate the signal from your videocard to both your monitor and fraps box

You would be capturing the entire output of your video card however and you would need to go to the trouble of editing out your other 4 wow instance windows from the video, unless you wanted to show them