i7 920

how much should i overclock this to?
i have liquid cooling

2.66 GHz core speed

2.93 GHz (Turbo)


i7 920 costs 300$
i7-975 costs 1000$
same chip under the hood..
but, they run on different frequency's


doing 4.0 GHz + with liquid cooling would be good idea?
or i shouldn't do it ? maybe start off at 3.0 and then its stable, then do more?