milehigh, you should also replace your stock bushing with metal (non-ball bearing) ones, especially since you're going with a M120S. You'll probably need new shims, too, since not all bushings are the same dimension and changing them means reshimming. Furthermore, your stock gear will suffer a significantly reduced lifespan pulling something like a M120S. Finally, you'll want to get either 1200mah+ and/or 9.6v minis to properly pull that spring.
Rough rule of thumb:
http://arniesairsoft.co.uk/?filnavn=...1c/springs.htm