i dont use one, but i have looked into it, the software is pretty darn expensive.
best way is prolly just get a cheap GPS from JBhifi or something, i work there and we have a few good ones for about 500$, dont have to worry about the laptop as its all in the one little unit.
another option though, is if you have a fancy ass phone (nokia n73 or something) you can get a little bluetooth gps transponder thing. and then you can "obtain" tom-tom gps software (---> bittorrent<---) and you get all the maps and stuff on your phone. i think the gps dongle thing is under 200$