Try searching through the Parallax site, they have several postings on how to use a Garmin eTrex coupled to a Basic Stamp. The eTrex can be acquired very modestly at eBay, where I picked one up for $60. It has flash firmware that is easily upgraded for free from the Garmin Web page. It supports the latest NMEA standards and communicates over a standard serial port using just Rec, Trans, and Gnd.
There's a long article where such a unit and a Stamp were taped to an RC airplane to log the plane's position every second. The article is complete with software listing (P-Basic) but with a little effort could be rewritten in Assembler or C.

Hal