Ok - so the critical sub systems for keeping balance, headings etc, are probably best served by a RISC type processor like the ARM and ATMega processors. But on top of that, you could put something that allows an easy access to image processing, communication, logging etc.. In the USAV project I use an Android phone (ARM 600 mHz) to handle the more high level stuff - such as navigation, telemetry, Twitter integration etc... Even though this system is remarkable fast when doing just that (a distance + heading calculation takes roughly 2 milliseconds), this system will be affected once starting to do image processing etc...
I just stumbled across this little thing, that surely would be more powerful and extendable.It has an 1.6 GHz atom processor and uses only roughly 4 watts. You can run any operating system on it and connect usb devices.
The size is only 3.50 x 2.36 x 0.63 inches (89 x 60 x 16mm) which would make it fit in many UAV's.