Is the USB mouse wireless or corded ?
If it's wired then you can buy a converter cable to plug it into the PS/2 port.
As you are getting problems with both the keyboard and mouse, then there could be a problem with the motherboard, but uninstalling/reinstalling those may be all that is needed.
As for the PCIe cards, a Google for those for Macs will produce a number of links to check out.