Determine your heat load. 100% of the computer's power results in heat at about 3.4 BTU/W. If the computer plugs in an ordinary wall outlet, a 4000 BTU unit will do, a 5000 BTU will provide extra margin.
The easiest option is to buy a small window unit and mount in in your 19" equipment rack and make a duct to direct the air.
1. The cheaper the A/C unit, the more noisy it will be.
2. All of the computer heat plus the "heat of compression" otherwise know as "super heat" will be added to your room.
3. You will need to modify the window unit with a condensate drain. These units direct the condensate to the condensor fan where it gets blown all over the place. You will also need a condensate pump.
Second option, water cooled unit such as:
Hope this helps...