You can use a wired GPS (also referred to as a mouse GPS), but I'm not sure which software would work best. On the 650, most people are happiest with TomTom, but I don't know if it works on the 600. I remember people using Mapopolis on the 600.
I think you should be able to find kits or components at
Semsons and/or
BuyGPSNow.com. Another place to look is
Seidio.
You can find a whole lot of information about GPS at
GpsPasSion.