How do you compute the AC capacity needed for a server room?

Just convert the estimated draw from Watts to BTUs: To calculate the BTUs/hour, multiply the watts by 3.413. 1 ton of cooling is equivalent to 12,000 BTU/hour.

Most of the energy drawn will be converted to waste heat.

From what I am reading, a pro would tell you to have someone come in and do the full calculation. That would take into effect any odd heat sources (solar for example) and your humidity concern.

In practice - I have just oversized mine by 25% or so in the past - to allow for new equipment. (In a small equipment room!)

I would also STRONGLY recommend you pick up this book:

Enterprise Data Center Design and Methodology = http://www.amazon.com/Enterprise-Methodology-BluePrints-Official-Microsystems/dp/0130473936/ref=sr_1_1?ie=UTF8&s=books&qid=1244469855&sr=8-1

Even though it is from 2002, it is still very relevant today. It is all about A/C, power, rack load, rack placement (cold aisle/hot aisle), good housekeeping, cabling runs, etc. for a datacenter.

Excellent reference book.