Look on the motherboard BIOS for the PCI-E settings, you can most likely increase the bandwidth there. However for an HD2400 you probably won't see a real impact in performance between x16 and x4 since it's not really a powerful card.
Not that I know of, that a BIOS thing usually. But don't worry, as I told you already, that HD2400 probably won't perform that much better under PCI-e x16 compared to what it does under x4.