Three alternatives to the stepperworld boards, which will also give better performance, are the Xylotex bipolar and unipolar choppers
http://www.xylotex.com and the Hobby CNC unipolar chopper.
http://www.hobbycnc.com
All will give you much more speed, and are much more efficient, than the Stepperworld drives. A lot of people use the Stepperworld drives and are happy with them, but IMO they're not the best choice.