As Khatharr mentioned above, you automatically get a boost from it. Yours is not the only application running. Having multiple processors means that even if your app is single-threaded, you still generally get one full processor to yourself.
There are many ways to take advantage of the processors without actually resulting to multithreading your app. Use asynchronous calls liberally. The operating system will do the work in the background and allow your application to continue to running.
If you do decide to write algorithms that require multiprocessing, remember that your game will likely need to be runnable on single-processor machines. Also remember that heavily loaded machines may degrade to single-processor performance and your program (not the bogged-down system) will be blamed for bad performance.
It is consequently unwise for mandatory processing to require multiprocessing. If you build your game such that it requires a quad core processor and for whatever reason all four processors are not available (including heavy system load due to all the malware) your game will perform poorly.
Building your game so that eye candy uses extra processors, sure that is fine. Throw all those extra visual effects onto the spare processors. Put the extra audio processing on extra processors. Don't put game-critical systems on those extra processors because they might not be there.