i dnt much abt graphic cards ,so wanted to know that as i recently had a 512ddr2 8600gt nvidia graphics card which is not functioning properly so shopkeeper is ready to offer me a 256dd3 card .should i take it with some charges applicable.
The shopkeeper is fooling you. Even though ddr3 memory is faster than ddr2, the 256ddr3 obviously has less frame buffer. Insist on a 512mb ddr2 or better still ddr3. It's here that the extra charges are justificable.
Well, depending on the card, the GeForce 8600 GT 512MB DDR2 may perform better or worse, if you mean a GeForce 8600 GT 256MB DDR3, it would probably be faster for lower resolutions such as 1280 x 720, but anything higher then that and you'd probably want to get the 512MB DDR2 version, if possible the 512MB DDR3.