i don't think you can "unlock" or increase/decrease the multiplier of an Intel's processor unless it's an Engineering Sample ,and there is no way you can turn a Celeron to a Pentium processor as well.

But, if you want to overclock it, it can be done by increasing the front-side-bus (fsb).

Eg. processor with the speed of 2800MHz (2.8GHz) = 200 (fsb) * 14 (locked multiplier)
you can increase the fsb to 250 which will bring you to the speed of 250*14 = 3500MHz (3.5GHz)

Overclocking will increased the processor's speed but it won't change the cache of that processor.

Evogenix