Get some of the cheap diamond hole saws off ebay from the guy in Hong Kong. Then use a decent drill, make a template out of wood or something, then drill it out. Drilled out my 75g and took about 10 - 20 mins per hole. I usually use some duct tape on the inner wall of the class, attach the template with a board on the opposite side with clamps, then drill while someone sprays water occasionally with a mister.
-Mike