HRM?

Lots of threads abound with recommendations

My recommendation:
With GPS: Garmin 305. It's a little big but go to a store and try it on. I think it's great (gives you distance, pace, etc)
Without GPS: Polar F6

My only advice is, just getting an HR monitor doesn't mean you're "training with HR". It's a tool and you have to know how to interpret the data both while out there and after the fact. There are some good books out there with more info about this (see amazon, Joe Friel, etc).

Good luck!

Lots of searching here, its preference, but so far I am a huge fan of the Garmin 305 which is now around 130 dollars.  The ability to do GPS, recording, HR and for another 50 bucks cadence, makes it a solid choice for triathletes IMO.  Posting up my review of the 305 tomorrow on my blog, just need to snap a few more pictures off tomorrow.  

I have the Garmin Forerunner 50.  I bought it at Target for 50 bucks.  It's usually 100 bucks.  It works great the only difference is that it doesn't have GPS.  You first have to know your HRM Zones to really train correctly.  I read an article that about it that gave a simple formula.  That's what I've been using.  I will try to look for it and I will send it you.

I've got the Forerunner 305 and love it.  It has everything I need.  Multisport feature is pretty cool too.