Difference Between ATI x1300/x1600 and X800 Etc.?

I am perplexed. What the heck is the real world difference between the these cards? For example, the x1300 and the x1600 seem to be very similar in performance. They can be had anywhere from about $200 to as low as $130 with rebates. Now, what is the difference between cards such as those and an x800 or x850 for example?

I am familar with pipelines, clock speeds, etc. but there seems to be so many cards that are pretty much the same on paper - until you plug them in and start using them.

I was looking to replace my what I believe to be fried x700 pro. I had purchased, but not opened, and since returned an x1600. What kind of performance would that have given me compared to my x700 had I installed it?

It seems that manufacturers are deliberately muddying the waters.

Thanks for any input.