Oooh, an interesting question, although it's probably not getting the attention it deserves in this section
To understand how this stuff works, it's necessary to distinguish between a "process" and a "thread":
THREAD: a sequential stream of instructions that a processor can execute. A thread completely monopolises a processor (core) until that thread is "pre-empted" by the OS so that another thread can get its fair share of processor time. That period is called a "quantum" and its on the order of 10 to 15 milliseconds.
PROCESS: A grouping of one or more threads. They share the same virtual memory, and the same security token, but each thread remains a distinct set of instructions.
At any time, there may be no threads in a read-to-run (RTR) state, in which case the processors run threads belonging to a special "idle" process, or there may be one thread, or many. An OS component called the thread dispatcher has the job of looking at all the RTR threads and "scheduling" them across the available processors. It takes into account thread priority, the processor where a particular thread last ran (there's cache advantages from staying on the same proc), and other factors to continuously make decisions about which threads get to run on which processors.
The dispatcher is blind to process boundaries. It only cares about threads. If a process has 17 threads and another process only has 2, the first one will get 17/2 times the processor love, everything else being equal. Processes that form part of the OS itself have hundreds of threads between them, and by the time you add all apps there'll be tens or hundreds more, but the real question is just how many of them are RTR at any time...
If you have a game that can spawn 4 or more computationally-intensive threads that are generally RTR all the time, all 4 cores in a quad-core proc should stay busy, although they'll sometimes be temporarily diverted to handle the OS threads and any other apps which may be vying for processor time. Otherwise, if the game only spawns 1 or 2 RTR threads - a much more common scenario - the other cores will sit and do very little other than background maintenance tasks.
The short answer is "get a quad core if you can easily afford it". Your life won't be made any easier or simpler, but sometimes you'll wait a little less, plus the machine will last and be relevant somewhat longer.