I would use this:
http://www.mini-box.com/M1-ATX-90w-I...8&category=101
PSU. it will provide a more stable voltage. I have also considered this several times but always wonder if its really worth the delay created byt eh serial port(sometimes the PIC will be faster) but i bet you will likely be using code that will swamp the on board PIC.
Of course you could always use this as an opportunity to learn how to make your own Power Supplies(Thats what im going to do for the system I'm gonna build into my car)