Hardware (CPU / RAM) for academic computing
 

I am trying to configure hardware specs for a research computer, possibly a cluster and would like to know what would be best the task at hand. This computer will be part of a lab and will be a server for at least the next 4 years. It would be nice to have it expandable as well.

Primary use : Ansys 13.0 CFD - CFX, Fluent, etc.
Mesh sizes : 10M+ for now, maybe 30M-40M in the future.
Preferred OS : Windows

I was looking at a single tower Dell T610 with dual Xeon X5675's and 96GB ram. However I was also wondering about a cluster of systems with an i7-2600K (most likely would have to be custom built) .

I have read posts here and the ANSYS parallel guide that it is not good practice to use all cores in a multicore machine because of memory bandwidth limitations. If possible can someone clarify that a little? Would there be little increase in using 3+3 cores in the Xeon vs. 6+6 or is just that it wont scale linearly?

It depends on your systems (and the software, of course) how it scales with the number of cores. Usually it's not linear, but when using too much cores, it can slow down the solution process.
On my machine it's fine with 4+4 cores (but Star-CCM+), efficiency is less than using only 2 or 4 cores, but still good.

The i7 are usually faster than the Xeon machines, but can't handle that much memory. Also when connecting several machines, it's not just the memory bandwith but also network latency and bandwidth.

For 30 - 40M cells, you will not need 96GB memory. 64GB should be enough, maybe it also works with 48GB (but that's not sure).