ah, got you.
ok, so do you intend this to be an add on for your basic stamp board or are you planning on programming your own microcontroller?
if you are building this without your basic stamp the microcontroller is proably the trickiest bit to decide upon.
everyone has their own opinions on the best one to start with but if you are looking for a "just the chip" type project, either an Atmel AVR or a Microchip PIC probably best suits you best.
the PICs have the advantage of having being on the market for some time so there is a large user support base of hobbyists using them in the internet. the down side is you will need to buy a (relatively) expensive programmer for them.
AVRs on the other hand are newer, come with free C compiler and can use a simple inexpensive cable for programming.
i won't go on about these 2 any more. try searching this forum for "PIC" and AVR". this has all been discussed before.
you can expect to pay under $5 for a microcontroller chip on it's own (either AVR or PIC).
after that a voltage regulator circuit to give you a nice clean 5Volts is a good start. (search the internet for "LM7805" voltage regulator.)
motor drivers will be usefull as motors will draw too much current for your chosen microcontroller to handle. (maybe the basic stamp has these built in?) you can either build your own from "MOSFETs" arranged to form an "H bridge" or buy an off the shelf motor controller IC.
have you seen Admin's $50 robot tutorial?
it might answer a lot of the questions you have: http://www.societyofrobots.com/step_by_step_robot.shtml