Ok first thing is BIOS has no control over the frequency of Mach, I suspect what you have set is the processors frequency. You will have to be careful with that as you may be over or under clocking it, it should be set to whatever your processors speed is.
Ok so if this is a new computer that doesnt work then have a look at the optimisation document. It should be on the Mach3 main site but you may have to search for it as it is a new site and I am unsure exactly where it is now.
Hood