IDK people. I have a 2600k and a p67 ws revolution in my main rig and getting to 5ghz wasn't super hard, however I lucked out and got a good chip. However, I built another rig with a 2500k and a z68-ud5 and it just wasn't as overclockable.
I would hope that if you buy a PC that OC'ed to 5.2ghz, that at least the processor comes from the best batch available. You know what I mean. Under Prime 95 that overclocked processor shouldn't exceed 72c.