The difference between PCI-E and AGP is not really that big, you only start to see the difference when it is SLi vs AGP.
In the future PCI-E will probably show bigger difference in performance and PCI-E cards are cheaper than their AGP brothers because they are cheaper to make obviously. It may not show a distinct performance difference now but its far better technology on the whole and allows you to upgrade to cards that are not made on the AGP platform anymore.
It is the same with 2x / 4x and 8x AGP. The difference between 4 and 8 is very minimal and sometimes not even present. 2x to 8x is a large jump and so is 2x to 4x, but overall the difference is really not noticeable and its mainly the Graphics Cards specification you should be looking at and not what platform the card runs on.
Your better off on PCI-E even if you dont see the difference now, upgrading in the future will be a hell of a lot cheaper.