Although it's still not showing that AGP 4x/8x-only mainboards have the "other" key tab, and that 4x/8x-only cards have only ONE key notch. Just like 1x/2x-only gear, only that the former have only the rear end key/tab.
So there's one more "red X" combination: 4x/8x cards (like Radeon 9600 series) won't fit 1x/2x-only mainboards.