We had a similar problem... the computer room is in the middle of our 6-room 1200 sq ft office, no windows, one ac duct on with the rest of the office. That rooms 8.5' by 12' (about 100 sq ft) with 9.5' ceilings. We now have 15 servers (mostly mid-range pc's but a few 8-core xeon boxes) running full time in there.

The solution came in several parts... first was to move any critical/primary stuff to our offsite servers and use the in-house boxes for backup and local-only stuff. We already had a cab at internap with some spare room.

Next we got a couple ceiling panels (its a false ceiling like in most office bldgs) that have a plastic grid for airflow instead of the usual cardboard foamy stuff.

We got rid of the nice locked, enclosed cabinets and got breadracks to increase airflow, and put two cheapy ($15) home depot oscillating fans in there.

If things get warm in the summer (i.e. today) we leave the door open for a bit and it cools down quickly. We're considering adding an exhaust fan/vent in the wall or ceiling, but since there's no exterior wall it's a pain, plus we'd have to deal with the landlord.

We probably only spent $100 and a day of labor, and it's "good enough."