The 5770 is a beast. Look at this, 3 displays at the same time on my 2006. The VGA monitor is via a mini display port to VGA adapter I got at the Apple Store. I'm sure this uses up quite a bit of GPU VRAM as well as GPU Cycles.
BTW. I got this in hopes of setting up a client monitor that is a mirror of my second display. If you option drag one monitor over another it will mirror. Here is some more info on that:
http://fcp.co/forum/4-final-cut-pro-x-fcpx/1570-how-to-setting-up-a-client-...
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T. Payton
OneCreative, Albuquerque