So in all, what percentage of the processors in the 360 is used to keep the game up to speed..
I don't know how much, if any, the processors help with graphics on the 360. The processors do the heavy lifting of the physics, AI, etc. The ATI GPU (Graphics Processing Unit) is the the one doing the graphics work. The 360 graphics gets a bump because of the unified memory architecture (can access 512MB of memory vs 256MB for the PS3), better use of bandwidth, 10mb of eDram to help with anti-aliasing, and relatively easy to program for over the PS3.
The PS3 was originally supposed to do all the graphics work using the Cell by itself. Sony slapped in a Nvidia GPU at the last second when they realized that wasn't feasible. Theoretically, the Cell's SPEs are supposed to be able to help with the graphics but bandwidth concerns, cache concerns, memory concerns, and the fact that the PS3 is difficult to program for is hurting.