well, its not that you can't overclock, its just with FSB higher than 1600mhz might kill your northbridge in a short while if you don't have proper cooling.
For most G31 chipsets you can only adjust the Vcore and GLT volts for cpu overclocking :laugh:
You should get 400x7 easily on the...
1600FSB gonna be tough for the MATX P5K, make sure you have northbridge cooling :)
Those G31 chipset not the best for overclocking, but you should be able to do atleast 1600FSB