Normally, when you connect an external display, half your video RAM goes to buffer your main screen and half goes to buffer the external screen. Normally this would result in a performance drop since you'll only have 32 MB of VRAM for each screen. However, most games don't support multiple monitors. When you start a game on the PowerBook, it will probably simply disable the external monitor (though I can't be sure, plus I think different games handle this in different ways), giving the full 64 MB to the built in display. Alternately, if you want to game on the 20" display, you should be able to connect an external keyboard and mouse to the PowerBook and close the lid so you use only the 20" display. This would also give you the full 64 MB of VRAM.
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