This may not be the answer you are looking for, but even though my mouse/keyboard are USB, I use the USB to ps2 adaptors and plug them into the ps2 ports instead.
This is because I personally think that ps2 is more responsive when your system is under load (e.g. games, programs loading, etc). I am no expert but I think I recall being told that ps2 ports are interrupt based, and USB is polled.
You will probably also find that linux will have an easier time of configuring the devices automatically...
Hope that helps!
(easy to try anyway if you have the usb->ps2 adaptors supplied)