I got a triahtlon bike last year but found I had a hard time planning my workouts so I asked for Mike Pigg's Triathlon PC Coach for Christmas last year. I found it did a great job of planning my workouts using my HR. You can even put in your races and it plans your tapers for you. Pretty awesome for a newbie especially.
I got frustrated though with my basic Polar HRM so my husband and I sprang for the new 725SX with the cycling speed sensor. I LOVE IT!
This thing estimates your max HR...though it was about 20 bpm slower than my actual max of 200. It also allows you to do 2 tests: one estimates your VO2Max and the other tells you when you've worked too hard and need to take a rest day or do some light training. I can upload my PC coach workouts (except the swim ones) into my watch and then when I'm done, download the results into the planned data on my computer. It's wild to see the graphed out HR changes over changing altitude and temperature.
Sally Edwards also has a great book out that talks about the different HR zones and explains how there are 5 HR zones and what each one helps you accomplish: healthy heart, temperate, aerobic, threshhold, and redline. It tells you how to calculate your max HR and how to take your resting heart rate. I read it cover to cover and really learned a lot.