I used the Radio Shack adapter as well
no problems. I downloaded all my old data from my old gps that was serial only, the uploaded to my new Garmin 76cx using the USB. No problems what so ever. I can't remember exactly but the cost was only $16 or so, definately not more than $30, and definately worth it for not having to re-enter all those waypoints all over again.
My adapter was a cable about 6' long.