Hello,
the 945 CPU has a locked multiplier so you can't overclock it as easily. If it was 955 BE (Black Edition) for example all you had to do is go into BIOS settings and increase the multiplier. You could still probably be able to overclock it but it would involve increasing voltages, which would lead to more heat for which you must have an aftermarket cooler to do as the stock cooler won't be able to keep up with.