I dont think so, especially if your GT is overclocked. In some games there is a huge difference (Farcry with HDR) and some there is not much at all.
If you get a newmotherboard, you will need a new CPU (I think because of DDR2). If thats the case, your FX-55 will not be able to make the transfer. Personally, I wouldnt wait, and would sell your AGP board, and video card, and get a much faster PCIe video card, and PCIe mobo. Or better yet... sell your current mobo, cpu, and video card. If you wait to sell the FX-55 till the socket changes, you wont get near as much as you would now. Such is the case with the FX-51/53's. Dead socket, and people have to sell them cheap. You could come out with a nice dual core Opty, mobo, and much faster video card for the money you would get by selling your older stuff. The overall setup would be faster, even better if you overclocked the Opty.