Well, for overclocking your Phenom II X4 955, look at this guide:http://www.tomshardware.com/forum/258573-29-overclocking-black-edition-guide
General rule - DON'T
use CPU overclocking software through Windows 7, unless you can't overclock through the BIOS because it doesn't support overclocking or is locked.
Adding more fans would be a good idea, overclocking can heat up your CPU quite a bit.
An excellent aftermarket CPU cooler would be the Cooler Master Hyper 212 Plus.