I observed a C-5A Galaxy flying at about 6500 to 7000 feet below the broken cloud cover in the direction of Dobbins AFB. We were traveling from Cartersville, Ga. to Rockmart, Ga. via state route 113. I told my 5 year old to look at this plane as it was the largest US made aircraft that I knew of. My wife also observed the C-5A. This described plane was coming at us which would lead me to believe it could be headed back to the AFB. It was flying very low and slow as often happens in this area. About 1-3 minutes after we observed the C-5, I saw an object much larger and faster than the C-5, flying over the broken clouds, it made a huge sweeping, and tilted, turn from right to left in the direction of where I last saw the C-5. As it would pass over clouds it cast it's shadow on the clouds and then would be in the clear between the clouds and the shadow looked almost like the craft without cloud interference. A fuzzy dark grey colored, long cigar shaped craft with very sharp edges, front to rear. The width got larger as it tilted to negotiate the large turn, and the craft was not entirely circular as one might expect. The C-5 A has a wing span of 222' 9"' and a length of 247'1". The height is 65'1". This object that I saw was at least double the size if not more of the C-5A. As stated, it was flying about double the altitude than the C-5, yet was still as described, very much larger.