Hi Smeggy,
There are a number of operations in Fusion where additional processor cores are used/benefitial (RayTracing), that said - generally a faster serial processing pipeline will make the most difference.. ie for a given processor architecture (ie such as Romley-EP or if a bit older Nehalem-EP Xeon) a higher processor and memory bus speed are likely larger contributors to performance than the number of cores.
By "dual quad Xeon" I take that to mean a dual socket, two physical processors with quad cores each (so a 8 core machine)? My home system is a IBM with a single-socket quad core Xeon 5500 (Quad Core) and a rather anemic Nividia Quadro FX 570 (only 256MB!). In my case my best improvement would be to move to a 1GB GPU..after that GPU improvements would have rapidly diminishing returns (in terms of performance) and my next best bet would be to move to a faster processor.
So in the case of a Nehalem Xeon (ie. like x5550 or 55xx) you'd probably be better off turning off hyper-threading and turning on Turbo mode (which will shut down one or two cores and overclock the remaining) in the UEFI/BIOS configuration utility.
The shorter answer is that a Xeon EP (midrange) quad core system, even a few years old, is plenty up to the task if you have a appropriate GPU (I think 1GB is plenty good enough) should be fine.
-Nathan
Nathan C. Skalsky
Portland, Oregon USA