Well said jdircksen, you will want to make sure the bulkhead is matched to the hole saw.
I recently bought the Dart Overflow Kit from Glass-holes.com for my 210g build and have been very happy with it.
The kit included:
acrylic overflow box with teeth and baffle
2 2" bulkheads plus 4 gaskets
Matched size hole saw
Threaded street elbows
Rubber tube for gurgle reduction
They also sell all of the parts individually.
Savko is a great source for plumbing parts. I am not sure if they stock the hole saws.
When drilling the 210g I stopped every few minutes and dipped the hole saw in water to keep it wet. I did it this way because I was drilling the tank with the bit vertical.
If drilling horizontal you can build a damn with plumbers putty to hold the water.
Put duct tape on the bottom side. this keep the glass plug in place until you are through all the way around and helps to reduce chipping.
When you get to the last little bit go very slowly to reduce chipping as well. You will always have a little bit, but you can minimize it by using light pressure and letting the bit do the work at the end.