i7-870 is a socket 1156 part that will be obsoleted with sandy bridge introduction.


i7-950 is a socket 1366 part.


Overclocking is done by raising the voltage to the cpu and thus speed. Downside is increased power dissipation and heat that if overdone results in instability and reduced life.  


An Intel Xeon cpu can range upwards of 8 cores and 16 threads. And costs far more than the $279  i7-950 4 core 8 thread  cpu.  


Take a running $39 500GB hdd and drop it on the table. Do the same with a solid state drive. Guess which one will still work...