My board still has the old 2x AGP slot, so I'm not even using the card to potential.
You know the speed of the AGP port does not really matter cos the 2x, 4x etc means how fast is the memory transfer between your card and the CPU. But since 2x is pretty fast you wouldn't notice much difference (if any at all) if you switched to a 4x AGP motherboard unless you are using very high resolution textures and/or high resolutions (above 1024*768). What does matter though is the speed of your processor. If it's below 1-1,2Ghz then probably it's holding back your card.