You might get more responses if you ask this in the Simulation/Flow and Administration forums.
The folks who use Flow will know about the best setup for that program. I believe that is does help to have more processors. A cluster may work, but I don't know at what point the slow IO between workstations causes problems. The Flow people can also tell you how much help a high end graphics card will be. I think I have heard that Flow is one of the few places where a high end card really is useful.
The folks in Adminstration can give you the best scoop on the fastest machines. SSD seems like a good idea for most people.