I believe that the newer mobos don't like AGP2x or lower at all. I remember there being warnings on some of the old Intel P4 mobos saying not to throw anything less than an AGP4x vid card in.
As for the AGP8x vs 4x, so far everything seems to be backward compatible with the 4x so as yet its not an issue.
This all has to do with voltages. Some crappy cards are not keyed propperly and will fit into slots that they are not compatible with. Some 4x cards want 5volts, but 8x agp is 1.5volts (i think). So you just have to be careful. If you use a recent card tho, you should be safe.
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As for the AGP8x vs 4x, so far everything seems to be backward compatible with the 4x so as yet its not an issue.
(And 2x is 3.3V... info from Abit site)