I work for a company called Cambiatta. We work in conjunction with a cyclist's cycle computer and heart rate monitor and capture the data that is acquired for download to software. I have a lot of expereince with various heart rate monitors, since we are designed to work in parallel with them. I definitely echo the other replies' advice: target which functions are important to you, and then get the most simple, inexpensive model that fits the bill. As you train more and use the HRM you choose, you'll learn a lot and in a couple of years may choose to upgrade. In the meantime, you want something that makes your training easier and more efficent, not something that requires a lot of your time, attention, and money. The functions and features I like are: the ability to set zones and downloadability.
There is no need to get a unit that does HR, speed, and cadence. You can get a cycle computer seperately.
Monitoring your heart rate is an effective way to track and improve your training, but don't get too sucked in. It's easy to focus too much on HR when you first start monitoring it, and forget about other things, like: how your body feels, what's going on around you, and how much fun you are having.
I would suggest looking online or at your local bike shop, instead of at Dick's. Online will offer you the most selection and your local shop will offer you the best service and advice. At Dick's, you won't necessarily get either.