The keyboard error message is probably coming from the BIOS. My last Dell Precision workstation had a USB keyboard/mouse setting in the BIOS that one could disable/enable fairly easily. My current IBM workstations have the same option.
The PCI USB card will quite easily handle your USB keyboard and mouse, and get you running with a faster V2.0 USB connection for cheaper than replacing a motherboard.
I've done the same for a couple of my IBM workstations, because they run power to the USB mice and keyboards when the computers are turned off. That drives folks nuts, because the mice and keyboards can really light up a bedroom with those blue LEDs and so forth. I also have several USB-switched power controllers, which turn off parasitic power to monitors, sound amplifiers, and so forth (Get a Kill-O-Watt meter and see how much juice an LCD monitor's power supply in standby draws over time!) They wouldn't work when the USB connection stayed live when the computers were powered down.
So I put PCI USB cards in them, and ran the keyboards, mice, and power controllers to those particular USB connectors. When the computers are powered down, there is no voltage from the PCI USB cards to the USB accessories. Voila'!