GeForce 8600 GT 512MB GDDR3 is one of the many variants of the fast-middle-class GeForce 8600 GT.
On this version, the GPU is equipped with 512MB of standard GDDR3. The architecture allows the use of 512MB, since the GPU's processing power is strong enough and the memory controller is up for the task of processing 512MB. Therefore, the performance is superior to the 256MB version and any other version of DDR2, including 1GB.
However, and due to its architecture limitations, modern demanding games will require reduced settings, even if they don't require more than 512MB of video ram. Very Demanding games like Metro 2033 are only playable at the lowest settings.