Another for HR + cadence - I also try to avoid keeping an eye on speed, although it happens. When following a cue sheet I also find elapsed distance helpful.

However, I do lust after a PowerTap, although it is out of my price range at the moment. A Garmin with GPS maps would also be pretty sweet especially because there are .tcx and .gpx files available for most of the brevets that I ride, and it would be great to have reliable elevation stats. That said, working on my cue sheet/map skills is probably a good idea too.