I have an old version of Streets (Streets 98) that I've successfully used
a Garmin eTrex Camo GPS unit with.  It worked pretty well in my casual
testing, but I decided I didn't like trying to balance a laptop up front.
This is the low-end eTrex without the map display.  I had to put it in
NMEA mode instead of proprietary Garmin mode first, but after that the
GPS control panel was able to talk to it.

This is somewhat more expensive than the DeLorme unit...it runs about $130
for the GPS unit, plus another $40 or so for the serial cable/power
adapter.

David Brodbeck, N8SRE
'82 Diesel Westfalia
'94 Honda Civic Si