You can get an idea of the relative performance between the various processors by using this handy comparison tool:
http://www.anandtech.com/bench/default.aspx?p=55&p2=61
Although the tool does not have the e5200 processor, I used the E6550 instead. Ignoring overclocking for the moment, and depending on what applications you run, you should expect the E8500 processor to be about 35 percent faster than the E6650 (Sysmark Overall). By the way, if you use the E4700 processor instead of the E6550 processor the performance improvement with the E8500 is 42 percent. The E5200 should be somewhere inbetween.
If you compare the i5-750 processor to the E6550, the i5 system is about 46 percent faster than the E6550 system (SysMark Overall).
As a first approximation, I would assume that all of these processors would overclock equally well, and the percentage increase in performance would be about the same. Only you can decide if these performance improvements are worth the cost.
If you are a gamer, oftentimes the biggest performance gain can be made by upgrading your graphics card rather than your CPU.