I've recently posted a "Work in Progress" Gadget on AeroXperience, that changes the current Vista Power plan based on currently running processes.  I wrote the Gadget because whilst playing Assassins Creed, I noticed Vista was lowering the CPU to 50% - which was dropping the frame rate by around 15FPS.

I normally leave the power plan on Balanced to save power, but got fed up having to manually change it to Performance whenever I played AC.

Download here.