I have yet to order my overflow...
 

and was wondering how hard it would be to drill my 55 gallon tank for a sump below? What size hole does one drill? I have a Eheim 1260 pump for the return on a 15 gallon tank.

Drilling is actually quite easy, you just need to make sure your glass isn't tempered. I would definitely recommend drilling for your overflow as it is much better than using one of those syphon type boxes. The size of the hole you need to drill will depend on the size of the bulkhead you want to use.

Be careful, alot of the 55s out there have tempered glass on all sides! Sometimes just the bottom!

If you know the manufacturer of the tank, then it's best to contact them first. If your tank can be drilled though, I would definitely recommend it. I'd do a minimum of 2 holes myself.

consider a Herbie (2 drains) then another hole for the return

I believe it is a All-glass aquarium (black trim and bracing in the middle).

I'm a newbie, what's a "herbie"? Two drains would be an advantage?

A herbie system utilizes two drains, one which the water will go through all the time, the other is used as an emergency drain. Then you'll need a gate valve to control the ammount of water draining down into your sump versus what is being pumped back up. This creates an almost silent system

As no air is drawn into a Herbie it is silent, with the only sound coming from where the water drops over the lip of the overflow.

once you get drilled, you'll never go back! lol