Can you switch your gpu interface to a pci-e 16x from an agp 8x without getting a new motherboard?

No, this is not possible. The chipset on your motherboard controls your interfaces, and if it's programmed to understand AGP instructions, it cannot use PCI express instructions. The chipset is soldered onto the board, and cannot be replaced. Not to mention you can't plug a PCI express card into a AGP port, or a regular PCI port, so you don't even have anyplace to plug the card in. In short, if you must have PCI express, and your motherboard isn't built for it, your only option is to get a new motherboard.