Impact of the L3 cache on performance - worth a dual-processor system?

As always with caching questions, the answer would be "it entirely depends on your workload". The cache is only of any use if your running processes are spending a significant amount of time accessing memory and exhibit a noticeable locality of reference for memory addressing (and are not happy with the smaller L1/L2 caches present per core for this matter).

Having a high number of processes running within different threads increases the odds for thrashing of the shared cache and thus diminishes performance gains which possibly would have been achieved otherwise. This is also the reason for increasing the cache size with an increased core count - the more memory-competing threads you have running, the larger your shared cache likely needs to be in order to be useful at all.

There is an oldish article from Tom's Hardware comparing two old P4 chips with and without L3 cache for a number of rendering / graphical workloads. The numbers are rubbish, as is the whole benchmark, but it contains a nice explanation of the caching architecture in general and L3 caching in particular.

The bottom line: you likely would not notice the difference, but if you need the exact numbers, you would have to purchase both CPUs and run your workload on both of them to compare runtimes.

People saying "a mere 20 MB increase in L3 cache" simply do not know what they are talking about. A sensible increase in cache size for a given architecture is likely to cause a sensible boost in performance, even with an average load. This is more true, when you think about the turbo boost architecture implemented in sandy bridge and ivy bridge processors.

I had the chance to experiment this personally in several different stages of the x86/x86_64 architecture: Sempron vs Athlon, Celeron vs Pentium 4, Pentium4 vs Athlon, Pentium4-m vs Pentium-m, Pentium 4 vs Xeon, i7 vs Xeon E5. Whenever the cache is bigger (usually doubled or almost doubled).

Whether the cost of doubling the cache is affordable, is up to you. But Xeon are better for stability, since they support ECC memory and such technologies, which are obviusly a must-have in certain applications (such as 3D simulations for aluminum die-casting, which is my case).

From your description of what you do and how your current system handles it I can only wonder why you want to replace it. At best the L3 cache would give a trifling boost but at a great expense but in your described use case you can't expect to see any difference resulting from a mere 20MB increase in L3 cache.