Need info on DDR2 and DDR3 Graphic Card

[eastsmile]

My Rig:
Motherboard:Gigabyte P35DS3R
PSU:Cooler Master 500watt
Processor:Intel core 2 duo 2.66Ghz
RAM:Kingston Hyper X 2GB
HDD:Western Digital 320GB

I plan to buy ASUS Nvidia 8600GT graphic card. And I need info, what's the difference between 512MB DDR2 graphic card and 256MB DDR3 graphic card,in term of technical aspect.