Well I was going to say that memory bus width is actually more important then memory clock speed, but I think all the cards you've listed are 128 bit, so it's a moot point...
You want to look at core speed first and foremost, but also consider the number of pixel pipelines available on the card, as that also has a direct coorelation to speed.
Think of it this way,
Core Speed * Pixel Pipelines = max theoretical speed (or fill rate, for those 'in the know') Thats literally how many dots the card can draw per second under ideal conditions.
Then your Memory clock speed and your bus width (wider bus = better) determine the max memory bandwidth available to the processor, and the memory bandwidth always limits the theoretical speed to some extent.
Thats a very rough evaluation, I know it's not perfect, but it should be a good enough guide to let you figure it out...
I'll dig up the old video card guide and post it if you want me to as well. It'll tell you exactly what all the stats are for each of the cards you're looking at.