As far as I remember his driver design is "Unipolar". To get more power from your stepper motors you should use a "Bipolar" driver. I guess it all depends on how much you want to spend and what performance you will get.
I started out with his first version. I still have it. It is very weak in my eyes but a good place to start. I also have a bipolar driver that I built from kit form. It to is week in my eyes (I like servo's now)
Each level you move up will cost you more! A large amout of people on this board like the "Bipolar" driver from www.xylotex.com As I stated above I built the Bipolar driver from Dan at Camtronics.
My advice to you is: If you can afford the Bipolar driver get it.