seanz2003:
good point. the voltage output from an adjustable switching regulator will sag with relation to the input voltage.
actually these switching supplies won't vary like that within a wide range of voltages. Sorry about the link earlier, the data sheet is actually here. The section on the characteristics of the variable type happens on p15 where it talks about calculating the resistors for the voltage feedback loop. In the ebay board this is replaced with a trimmer pot so it can be dialed into the voltage you desire.
My primary reason for suggesting a fixed one was so the OP didn't need to do the voltage setting step granted 5V is the only level needed, and that this is one of the available fixed-output variants.
Geoff