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I wouldn't worry about needing 200 amps over 150 unless you are running serious power tools and need lots of circuits for them. We had central air (in the desert, no less!) when our panel was only 60 amps, and we also had an electric range, dryer, etc. We would only pop the occasional breaker when we had the microwave, fridge and air compressor for tools running all on the same circuit simultaneously. The AC was never a problem.
spoke to an electrician friend today. also confirmed all i need is 150.......200 is nice, but 150 will be fine base on what we need and plan to do in the future.
You need to understand just how much full load amperage, the AC unit will draw, and start calculating the amount of amperage that all of your other electrical appliances will demand when active.
Once you are aware of the usage required while maintaining a balanced load on the main, you will be able to determine whether or not you will have a problem. My advice would be for you to install the
central air unit, and put it into your electrical panel as an independent, dedicated branch circuit breaker, in line with an amperage value equal to, or slightly greater than the AC will demand. You may also need to brush up on Ohm's Law!
Actually ohms law in its basic form does not work for ac power as it applys to dc.
For ac you have to substitute resistance with impedance values and that adds a whole new set of calculations that are needed to be worked in.
As already stated on this post, many home buyers look for 200amps when buying these days. If you aren't selling, 150 should be enough for your needs, though if you have personal computers that draw a lot of power (high end graphics cards tend to do this) and a lot of electronics + microwave + refrigerator + range + freezer + washer/dryer, all running electric, you might want to splurge for the extra amperage. These days, modern electronics are everywhere and tend to draw a lot of current (even if they use less than they once did). Make sure your lines are balanced.
When I bought my house 3 years ago, I had it upgraded form 100amps to 200amps. Combined with adding a new 20 amp line to one room of the house, installing a new mast (the old one was falling apart), and installing two additional 20 amp lines in my basement, the total cost was $3000.00.
my house has 100 amp service. it has an 240v electric stove, 240v electric dryer, 240v central air, forced hot air heat and a large in ground pool with a 240v pump among all of the normal electric loads. The main never pops, the breaker box doesn't hum or get hot. If you have 150 amp service you will be more then supplied for what you want to do. I am upgrading the service to 200 amp as we speak, but only because we have done major renovations and rewired most of the house.
Think about the reality of modern appliances, the ones from today draw half or in some cases a third of what the older ones did, and until 15 or 20 years ago 60 or 100 amp service was the standard in any house. 20 years ago central air units were huge electric draws, but now they are preety low draw with minimal start up spikes.
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You need to understand just how much full load amperage, the AC unit will draw, and start calculating the amount of amperage that all of your other electrical appliances will demand when active. 
