There is no cap on increases to property value. The City of Atlanta and Fulton County can decide to assess your home at any level they want. Currently legislation is being considered that would cap increases in property taxes to the lesser of 3% per year or the rate of inflation. However, Mayor Franklin is pushing to increase property taxes in 2008-09 to pay for budget shortfalls. It seems like there is a 50/50 chance of this succeeding.
Regardless, aggressively manage your tax assessment and take every opportunity to appeal it if appropriate.
You might want to have your REALTOR pull tax comps for nearby properties to get a 'real' assessment of what the taxes may be... you may close at the 'reduced' rate and be in for a HUGE suprise next year.
As mentioned, there is not a percentage cap of the amount taxes can be increased in a year. But, the tax assessor is not supposed to be 'chasing the sale'... there's supposed to be an element of neighborhood uniformity.
Matt, there is no maximum that a property tax can go up in any given year. They make their assessment based on what they perceive to be fair market value. This market is making them nervous because values are going down and homeowners are fighting their tax bills because of this. If this is not a new home you are buying, my guess is that the value that bases your taxes was termporarily reduces for some reason. You can call the the counties tax assessors office to ask them what this means and I am sure that they will share with you. This will be very valuable information for you. Hopefully, you will share this information back with us. It is always great to learn something new!