Hot times are coming and you may find yourself buying a new air conditioner, but how do you figure out how many BTUs you need? Silencedotcom over in the Consumerist Forums posted this BTU guide:

Length of Room x Width of Room [in feet] = Area
(Area x 30) + 2000 = BTUs you need

UPDATE: This Energy Star chart is what you need:
• If the room is shaded, reduce the BTUs by 10%.
• If the room is very sunny, increase the BTUs by 10%.
• If you plan on placing the air conditioning unit in your kitchen, add 4,000 BTUs.
• If more than two people will regularly be in the room (e.g., an office), add 600 BTUs per person.

We're no A/C experts but that formula roughly jibes with the number of BTUs Lowes.com recommended for our apartment size. — BEN POPKEN

Properly Sized Room Air Conditioners [Energy Star]
Little air conditioning lesson for everyone [Consumerist Forums]