Just bought a Garmin 800. I used to have a Garmin 305 w/cadence sensor and heart monitor. Can I use my 305's cadence unit successfully on the 800, or do I have to buy/install a 800-dedicated unit?

Both the speed/cadence sensor and HRM should work fine on the Edge 800 and 810 (plus many of their other GPS units). Garmin is still using the same old GSC 10 speed/cadence unit on their latest products. And, though they have updated their HRMs a couple of times, they all still use the same ANT+ protocol, so the older ones still work (not that Garmin HRMs are known for stellar performance to begin with, but that's a different discussion).