Garmin 500 or 705. The 705 if you want/need on-board maps and turn by turn guidance. Even with the 500 you can plan a route, download it to the 500, and follow it on the screen, though no roads will be depicted and there are no turn warnings or instructions.
And since you got a Madone, you're going to want the slick Duo-Trap ANT+ wheel and cadence sensor that mounts in the left chainstay. It's speaks Garmin.