It is believed that Samsung will be using its own Exynos processor and the rumor is strong that the choice will be of the quad-core kind. It is most likely that the processor that will be in charge of taking the computing burden for the Samsung Galaxy S III is the new Exynos 4412, which is 32nm quad-core design with ARM Cortex-A9 cores running at speeds up to 1.5 GHz. Samsung is already testing the new processor according to some sources.
The Samsung Galaxy S II with a dual-core CPU is already fast. Imagine how fast the Samsung Galaxy S III will be if it had a quad-core CPU.