Graphics Card Help?

Ok, first off, I have a Gateway 500 xl. This computer only has pci and agp grahics card slots. I was wondering, what will happen if I put a pci-e card in the agp slot. Is there anyway that this can mess up my computer. What will happen. If it works will a pci-e work better than an agp video card. Any help/insight you could give me would be greatly appriciated.