again.. it is not worth changing your mobo just to have a PCI slot. You will not notice any difference in performance between a PCI-e and an AGP card. Just because a faster interface is available doesn't mean that the rest of the system can utilize it.
Your CPU socket type is what is limiting your performance, not your graphics card interface. When you upgrade your mobo, upgrade your CPU as well. (you can upgrade more than just your video card with $450).
Until you are ready to upgrade your CPU as well, then just get a nice AGP card.
..and yes, changing your mobo will require reinstalling windows along with all your device drivers (you may be able to perform a "repair install, but that often leads to an unstable system)