You're probably best having your external monitor set to 640x480 and your Cache size in MixEmergency (the Advanced panel of the Preferences) set to around 400 MB. If you have videos hanging/stalling while you're performing, the most common problem is that your videos have been encoded to have large distances between keyframes. Re-encoding the problem videos to have keyframe distances of around 20-30 frames could fix this. Another possiblity would be that you're running out of free memory - which reducing the Cache size in MixEmergency might fix.
