unlikely to notice much diff, onyl really shows if you play games at low resolutions (ie 800x600 & lower)... a couple of fps lower at most or mebbie 300 3dmarks... somebody out there may have tested this... but the diff is marginal at best when U run 1024x768 & higher...
dont worry about it, you can alway buy a new board with 8x AGP if you think it is actually worth it... i dont think it is...
no-one shouts louder than someone who is being ignored, or in the case of techies, to be heard over the noise of their PC's ;-)