Mike is correct on the sampling times, however I have made things work well for this application at a much lower rate....basically whenever it gets to it. This can lead to some choppy response on the throttle and steering servo, but nothing bad unless you really load down the processor doing other things.

The Propeller is a better chip, but look carefully at the overall cost of all the cogs and infrastructure compared to a PIC system....might still be cost effective, never used one.