You can try, but I doubt it.
Slot 2 motherboards do not have any other FSB implemented apart from 100MHz, because it's just not reliable in a server class system.
XEONs also carry an onboard APIC that sends out diagnostics to the motherboard, so if it was overclocked, it might start doing all sorts of funny things. I would suggest that you avoid it if you wanted to overclock.