If you are running Vista, go dual core anyway. It takes up that many resources.
As to single threaded games, believe it or not the dual core will still be faster. For one, most dual cores use clock cycles more efficiently than the old single cores, so even if only one core is running, its still going to be quicker. Also, excess load from the operating system and other programs will be offloaded to the second core (which is not active under single thread programs), so it frees up one core to work much harder on the game or app.
In other words, you have no reason to purchase a single core processor. At all.