AGP 4x and 8x is electrically the same thing, 8x is just an extra mode on top of 4x. All 8x equipment must (yes must) be backward compatible to 4x mode.
So if you have at least 4x capability on the slot, an 8x card will work in there, engineering screwups aside.
Now, chipsets. There have been no K8 chipsets that didn't at least have 4x, so I'd say go ahead. The slots are mechanically keyed, so if a card/mainboard combination wouldn't work electrically, it won't mate anyhow. (But yes, there have been keying fsckups as well.)