I am very curious why the memory space has been included into the graphic card. Why do we have 512MB and 128MB. And what is DDR1, DDR2 and DDR3?? This questions are keep puzzling in my mind for many years and never get a clear answer . Many of my peers told me that the most expensive one is the best . I find this answer a bit lame. So can anyone tell me how to choose a good graphic card?

i roughly can spend 500 now with my tight budget. but i need to know how to choose a good one before i make any wise decision.