i think getting new mobo make me more affordable for the graphic cards.
agp cards are a lot more expensive than pci e of the same GPU
so i'm thinking new mobo
But you are going to spend more on the motherboard and GPU than just the GPU in AGP flavor to get the same end results because with a P4 and HD4650 the CPU becomes the limiting factor.
You could though take the change and get a cheap used board of ebay and if you are planning a new build in the future you could carry the graphics card over.