Air Conditioning - What size and brand A/C should I get?

I just purchsed a 2000 sq/ft home in the East Bay area, outside San Francisco, that has no A/C. It has a York Diamond 80 furnace, model #P3DNB12L08001A, and what looks like an evaporator box under the furnace (downdraft unit). I'd like to install a 4 ton unit but have been told that the furnace is too small for that and the biggest they can install is a 3 ton unit because of the CFM. The HVAC folks I've had in here also tell me I should replace the furnace but I think they are just trying to use me as a cash cow. The average price to replace the total unit is about $9k and to install just the A/C is about $5k. What do you think is the right size for the home and what brand unit is going to give me the most bang for the buck?

A load on the home will have to be done to know what size unit to go with. The company that can install the unit correctly and stand be hind there work is the company that you should go with. For the most part equipment is about the same.

HVAC Software, HVAC-Calc for Heat Loss, Heat Load Calculations (http://www.hvaccomputer.com/)

$49 well spent.

Get HVAC-Calc and calculate the cooling load or find a contractor to do it. My guess is 4-tons is way to much for that house in the bay area. When you get your load calc let us know if I'm right.