For example, the reference Nvidia GeForce GT240 on GDDR5 is (Green coloured bar) is:
550/1340/1700Mhz for core/shader and memory
Now look down and see the ZOTAC GeForce GT240 1GB ZONE Edition GDDR3:
550/1340/1580Mhz for core/shader and memory
The difference in memory gives you 16.5 vs 12.9 fps, a wooping 28% different. I think this is very convincing.