Yes that http://www.pololu.com/picture/0J2223.600.jpg
seems like a nice little board. It supports the raw input to the H driver. So you still need to supply raw pwm signals to the input of the beard. And the native pwm frequency of the arduino is about 480hz if I remember correctly. This is rather low for driving motors. I works great but it is a rather noisy tone. Although Much more expensive, this is also a nice dual motor board which comes with good documentation and software: http://www.robotshop.ca/roboteq-ax1500-robot-controller-1.html
It has a nice set of input features. You can input serial. Analog and RC meaning that it takes a servo signal. This could be interesting for you since you only need the arduino to output an single variable pulse width 50hz signal to control speed and direction. So you can use the arduino servo libary. Servo info: http://www.seattlerobotics.org/guide/servos.html
The board also supports potmeter feedback. So with that you turn a big motor and a potmter into a very high power servo. The board deals with the PID http://en.wikipedia.org/wiki/PID_controller
which is handy. Since it can all be adjusted in the included software. If you were to code that PIC yourself in the arduino environment it would be a big task to get right.
You need to build your own H bridge.
you could use the Arduino to control an H bridge IC driver like this:http://www.intersil.com/data/an/an9405.pdf
And use mosfets like these http://www.irf.com/product-info/datasheets/data/irf3205.pdf
Also note that you need proper high voltage protection diodes to protect the mosfets and driver ic's from high voltage flyback/inductive collapse spikes.
You should probability plan for much more than 160watt.
Dc motors can have very high amp draw while startup.
And when you stall them the only current limiting factor is the motor coil resistance which could very well be below a few ohms. So if that motor has a 1 ohm coil and you supply 24 at 100% throttle the watt could reach 576watt.
I was looking at using mosfets, but honestly I think I'm getting way beyond my knowledge if I'm designing the circuit for the motor control. I was looking at Pololu's high-power motor drivers, would these be able to do what I'm looking for? http://www.pololu.com/catalog/category/82
Or would it be easier to interface to the arduino using the high-power simple motor controllers through serial? http://www.pololu.com/catalog/product/1383
This is my first robot, but I'm jumping in head first since it's part of my senior design project.