the answer is 16.
A group at the University of Massachusetts at Dartmouth hopes to use a cluster of 16 PS3s to run the simulation, specifically looking at the properties of the gravity waves emitted by the collision. The cluster -- nicknamed the PS3 Gravity Grid -- was built with a partial donation from Sony, who must like watching really big stuff collide as much as we do.
Similar simulations have been run before (most notably by NASA), but this marks the first time the super-calculation has been performed using a game console. Like our colleagues as PS3 Fanboy, we admit that most of the technical jargon presented here is way over our heads. We're sure some of you will "get it," though, and not just resort to Tootsie Roll references.
[Via PS3 Fanboy]