It depends entirely on your monitor and it's native resolution. Most TFT and LCD monitors these days are 1280x1024. I have used it briefly with my TFT, but unfortunately you'll find plenty of 360 games either have borders or stretch themselves to fit the contents of the screen meaning they're at the wrong aspect ratio. That's why I haven't had my 360 setup for weeks - stubbornly waiting until my HDTV comes back.
To answer your question then. 1024x768 or 1280x1024 if it isn't widescreen.
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