AGP X1-X8 is the bandwidth of the slot, there were 3.3 - 1.8 and .8 voltage AGP cards and they were sloted so you could not install it in the wrong slot. If you had a X8 AGP card that was sloted to run in either X4 or X8 it would be backward compatible so it might run alittle hotter because of the voltage but has nothing in common with PCIe. As far as PCIe X1-X4-X8 and X16 is how many PCIe lanes the slot has to work with.
http://www.ertyu.org/steven_nikkel/agpcompatibility.html
http://www.ertyu.org/steven_nikkel/agpcompatibility.html