Well, DDR refers to it's bandwidth. every card has a given memory clock speed and p.e: if the memory clock is 1000 Mhz, DDR will ue just that, but DDR3 will double it, so it would be like 2000 MHz effective speed. Now, DDR5 doubles it again, so the final number would be 4000 Mhz effective memory speed. Always you must look for a DDR5 crad if budget is not a problem, and i'd recommend Nvidia 780, Titan (more expensive) or Radeon 7970 GHz Edition. Those are the best on the market right now.
PS-Nvidia is better, and you will get higher speeds there, up to 6000 plus Mhz.