i am running a fx5200 on a 4x agp board.
it works fine, and there is no reason to upgrade the mobo just for that.
and yes. the 6600gt is a lot better card, but i think parimal already knew it. a lot of people consider a product after taking a lot of things in consideration, including price.
recommending a 6800 to someone who is non a budget of 5700 is like asking a maruti buyer to buy a merc. ok, so not exactly that....but kind of
enjoy ur 4x card for now.....when u do get a mobo upgrade....that will change to 8x. don't sweat over that