640x480 is fine - like you said you want the fast frame rate and pixel size.
You can print reasonable size still - easily 6"x4", and if you have a really nice night of seeing, the image can be blown up to a little larger.
But what most planetary imagers do (and what I did when I was active) was make a composite of several images on a single 10x8" canvas (for example), so each individual image wasn't blown up to a large scale, but the sum of them all gave you a nice progression or series.
eg:
http://www.mikesalway.com.au/2009/01...ter-in-motion/
http://www.redbubble.com/people/mike...ynamic-jupiter