Thanks to no more incompatible Treo 300 headsets. Treo 600 in-hand, I went to Radio Shack and tried some normal ones out. After both the Plantronics and Jabra units, over-the-ear with a boom mike (low volume to listen, and really poor, tinny microphones), I tried the RadioShack brand "over the ear Telephone Headset" (#43-1905). Instant success!

Earphone loud enough for driving in my convertible S2000 with the top down at road speed, and boom microphone sensitive enough that my mom wanted me to make it quieter (just move away from your mouth). Additionally, the microphone has a foam cover, so most people don't hear wind noise and don't know I'm in a convertible.

Only bad part - difficult to put on my ear with one hand, but comfortable once on. Maybe I'll try the headband type...