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Typical OC kinda depends what your cooler is. Stock, S1283, True, etc?
However, with a high end cooler [read at least a S1283] I'd go with these:
E5200=3.6Ghz avg, 3.8Ghz if you really push
E7300=3.7Ghz avg, maybe 4Ghz if you really push
E8400=3.9Ghz avg, 4Ghz+ when you really push, the E0's will sail past 4Ghz
E8500=4Ghz avg, 4.2Ghz if you push
E8600=4.2Ghz, 4.4Ghz if you really push. To address the OP's question 4Ghz is literally guaranteed
The average factor is tough to determine because some guys won't pump 1.4v and some will pump 1.45v.
334x12 = 4GHz. Putting that high multi to use! You don't need an awesome northbridge heatsink if you don't do 400+ fsb.
Only problem is the low cache. The E7300 and E8600 are both 10x multi, so they'd require a board that could do a 400mhz FSB easily.
E8500 and E8400 would require an even higher FSB speed = need for a higherend board.
It comes down to 1) what can you afford. and 2) What is this system going to be used for?
E5200 + Cheap and good OCer P45 Motherboard = Cheap 4GHz but low cache
E7300 + decent 400MHz FSB board = More expensive, still low cache, 4GHz.
E8400 or E8500 + 400-500MHz FSB board = expensive but worth it. Fully featured motherboard, good amount of cache, 4GHz.
E8600 + decent 400MHz FSB board = CPU is expensive, but mobo could be skimped down on. Other benefits of above build.
I'd go for the E5200 if you want a cheap 4GHz. Go with the E8500 if you want all the goodies but w/out the killer price.