What model where you looking at? And what are the specs on your current system? What do you do, mainly gaming? GDDR 5 is currently the fastest type of video memory available on newer graphic cards, as well as many other variables. There's pixel fill rate, texture fill rate, model of GPU (Graphic Processing Unit), number of stream processor units, the list goes on. A good site to learn more about video cars is:
http://www.gpureview.com/
Here you can also compare video cards side by side. Any other questions just ask.