Adding GPS to Wardriving is like adding cheese to a pizza!
It makes the whole wardriving experience more enjoyable, as you can see where the AP's your picking up actually are on a map in KisMAC or MacStumbler(see below). It is also very handy when you want to find where a specific Access Point is located so you can make sure that your pointing your cantenna in the exact right direction making you pick up more Packets.

Here is a list of compatible GPS systems that will work with KisMAC & MacStumbler software:
I personally use the Garmin ExVenture with KisMAC on my G4 iBook and it all runs smoothly. I also had to purchase a serial to USB adapter cable from eBay to be able to plug the GPS into my Computer. (you will probably have to do the same, as most GPS's still use serial port connections and not USB.)