Sorry ...Hi gesilvia1 & welcome to Windows 7 Forums
DDR3 has several advantages over DDR2 memory, such as lower operating temperatures, lower latencies, and reduced power consumption, DDR3 memory is more costly and not compatible with DDR2 sockets on a Motherboard. Memory on a GPU is not interfaced with the Motherboard Slotted Memory.
As to your second question - there are many types of Graphics Card, so it depends upon what you would be using the Computer for.