not that im a skeptic, but by how much is the increase in performance? is it noticeable? im using a geforce fx 5200, not really a power card, but dx 9 nonetheless :)

Here is a thread from guru3d re agpx4 vs agpx8 which may answer most of your questions.
The short answer to your question would be 'not very much'

http://www.guru3d.com/forum/showthread.php?s=&threadid=64119&highlight=agp4

Hope this helps