I don't think there is any difference at all between AGP pro and standard AGP. If there is a difference, its something technical I probably wouldn't understand. But i'm pretty sure they are both compatible with each other.
As for the different formats, (1X 2X 4X 8X) they are just to do with the speed of the card. Needless to say, 8X is the fastest, but an 8X card will work in a 2X speed slot, but it will run at 2X only. It works vice versa as well, a 2x card will fit in a 8x slot, but will only run at a maximum of 2x. Hope this kind of helps. Anything specifically you wanted to know?