small programmable board/box

[whitegoodman]

I'm looking for a small programmable computer/circuit board/etc. Doesn't need to be very powerful, but does need some form of flash memory or small hdd, etc. WHen I say small I mean as small as it can get, PDA size/home router size/etc.

Preferably as cheap as possible, couple hundred bucks at most. Needs to be versatile in it's external interfaces, or just capable of adding any external interface I want with some hardware modifications. Memory/drive size just needs to be big enough to store a program, and run it, that interprets voltage outputs and displays them on a monitor with a graphical interface.

Any suggestions would be welcome, even if I have to build one myself, any help with what board and components to use.

Thanks a lot