Tire pressure will depend on the course. At Big Bear in the DH racers run somewhere around 16 to 20psi. at another course we run around 28 to 30psi. I do not know what the xc guys run since I do not race xc.
The best way to find out, when you get to the races, talk with one of the expert/semi-pro/pro riders and see what they are running. They are usually more then happy to help out.