To get a home loan, the property will be appraised by the lending company prior to loaning you the money for the house. The company will only lend you up to the appraised value of the home. So, say you want to buy a $200,000 house, and it assesses for $195,000. You'll have to cover that extra 5K up front (if the sellers wont negotiate). There have been a lot of changes in the appraisal process since May 1 due to the mortgage crisis to make it a more even playing field, but a side effect of that has been that a lot of properties are appraising at below their asking price, which has caused a lot of deals to fall through.
The county does their assessments for property taxes every few years, I think. Go and look around on the fulton county tax assessor's website. The 15K homestead exemption is taken off the top of that. This website explains how it works:
Real Estate Taxes* Figure Your Monthly Property Tax
One more thing: the appraisal done by the lending co. is much more in depth than the one done by the county. The actually go in the home and can assess if you've done renovations, etc, to get a better idea of the value, but the county does not.
I hope this helps.