Here is my Xeon Overclocking experience. I did not do these trials systematically so these numbers are from my memory.
I used Phoronix "Build Linux Kernel" benchmark for comparison. My AMD-1100T based system had a score of 150 and that is my baseline. AMD 8370E scored pretty much the same as 1100T. My i7-4790k machine scored just under 100 for the same test. Score is basically time in seconds hence lower the better.
Edit: All these results with default CPU voltage.
E5620 (4 core 2.4GHz) overclocked to BCLK=170MHz, CPU ran at about 3.2GHz score: 180 and that did not meet my expectation.
i7-930 (4 core 2.8GHz) overclocked to BCLK=180MHz, CPU ran at 3.60GHz score: 140
(While running Intel Burn Test in Windows Vista the system crashed a couple of times and I had to bring multiplier a couple of notches down so the score might be a little lower now).
The performance of an overclocked i7-930 seems to be very similar to 1100T running within specs.
All in all, considering the money I spent, I think it is better to buy a faster CPU/Mobo kit and stick with the default clock unless you already own an overclockable X58 motherboard. It may make sense to extend the life of a setup by overclocking. Overall, purchasing a used X58 board for overclocking purpose will have much diminished returns. My i7 4790k+Motherboard was about $320 from Microcenter. Compare its performance with my used Gene III setup that was $230 from ebay.
That being said, it is certainly a lot of fun!