Yankee:
I have used those Pololu drivers and found that they really can't handle much more than 1 amp without a heatsink and it's very difficult to attach any kind of effective heatsink to them. I would reccomend these: http://www.automationtechnologiesinc.com/products-page/kl-stepper-drivers/kl-4030-24-40vdc-3-0a-microstepping-driver. I've been using them on my CNC router for several years with absolutely no problems. And MarkT is right, use at least 24 V.
Oh, you need a cooling fan, that's a given... something like a 4 or 5cm square cooling fan blowing through a stack of the pololu controllers might work nicely.