2×Xeon E5345s gives you 8 cores and 8 threads running at 2.33 GHz.
Given the fact it was launched in 2007, its multi-threaded performance should be equivalent to that of an i3 at the best.
Also you have to remember that most games don't utilize more than 4 cores. Therefore the effective performance you get for gaming will probably be equivalent to that of a modern 2 core Pentium.
In terms of single-threaded performance, this 2×Xeon setup gets a massive hit.
Hence, I would say that if you are aiming for 60 FPS in games that take advantage of 4 or more cores, it should not be a major concern. But in games that rely on a single thread, you may see a loss in performance.
I would also recommend a single GTX 960 against dual GTX 660s.