A/C Returns, More.....

We have a brick home, well insulated, of about 2,000 sq/ft. Our cooling unit is a 4 ton Carrier Infinity unit.
I only have one return, 20"x25". I've been told that I should install (or have installed) a second return on the other side of the house.
Also, what's the max vacuum, or whatever the A/C tech term is, for the inside of the return of an operating unit ? (I'm sure it varies with units, but surely there's a "ball park figure").
Lastly, we have a "computer room"/study that's almost right under the A/C unit in the attic; dimensions 12'x15'; it stays about 8 degrees warmer in that room than the rest of the house. There's a large UPS unit, and three PC's that run continuously , and one of them is an older dual Xenon using the older (and much less efficient) dual core processors. Should I install another vent in this room ?
Any further info gladly provided. Thank you very much.