Most GPS hardware sold today uses standard protocols that can be recognized by most GPS programs (though Co-Pilot was using proprietary hardware last time I looked).
Based on your mention of Tom-Tom, I assume you're looking for a car GPS system. I would strongly recommend you visit GPS Passion - http://www.gpspassion.com/ - they have reviews and comparisons of all the major GPS programs and hardware. My personal favorite is Intellinav. It uses NAVTEC map data, which seems to be the most up to date, and the user interface is great. They are coming out with with V 2.0 soon, which promises to be even better. (free upgrade from v.1 if you buy now)
As far as hardware goes, I've been using a CF GPS unit on my Dell Axim, but plan to switch to a bluetooth unit when I get my HP 4155.