Just out of curiousity I've heard there is not that much difference, but I've also heard it makes all the difference in the world...so, which one is it?

These series of tests show that 8x AGP has no appreciable advantage over 4x AGP:

http://www.hardwarezone.com/articles/view.php?cid=3&id=473

http://www.hardwarezone.com/articles/view.php?cid=3&id=562

http://www.hardwarezone.com/articles/view.php?id=604&cid=3&pg=1