keep in mind too, that just because an overflow is rated at a certain amount, lets say 1200gph, it may not actually perform at that level. You can do a simple test once all the plumbing is in place by allowing it to flow into a container that is easily measured. Use that to calculate the actual gph flow and then you can make a more informed choice on your return pump. Example: I have a overflow that is rated at 1200 gph, and a return pump rated at 950 gph (about 800gph when you consider the 4 ft it sits below the top of the tank) and it still outperforms my overflow. I have a tee fitting with a ball valve on it so the excess gets put into my fuge. It may sound confusing, but really isn't and its a lot easier to understand once you have it set up. Unfortunately that means your spending money before you know all your system details. There are calculators out there that supposedly calculate the amount of water a given pump will give you if you use "x" diameter piping, have "x" number of elbows, etc.. but I haven't seen anything about them to show how accurate they really are.
As far as drilled or not, just like salty said, drilled usually looks better, but costs more. One option if your somewhat handy would be to make your own overflow and drill the tank yourself (the back glass as the bottom is usually tempered and will shatter if drilled), you can pull it off cheaper than a drilled tank is new and you can design your overflow how you want it to look.
BTW- if you put overflow box into a search engine like google you will get may sources explaining exactly how an overflow works. AS well as tips on how to set the system up with an overflow box.
