Go into the BIOS and change either the multiplier (if you can), or the front side bus (should be at 133 now). Raise the voltage (vcore, should be at 1.75, I believe) if it's unstable at higher speeds. Watch your temps, don't want to get too high. At any signs of instability, go back down in speed. Go in small increments so you can find your exact limit.
Don't put it straight to 145, that's way too big a jump. Move as small as possible, boot into Windows and run 3DMark2001 once. When you start getting errors, up the voltage and/or go back down in speed. Eventually you'll find the highest stable speed that doesn't overheat.