You can't really be sure that each E-2040, E-2060, E-2080 is as much overclockable as the other one of the same type. Actually it's luck, what you get.
E-2040 etc. processor chips aren't produced to become E-2040. The are produced to become any E-6xxx core duo processor with conroe core. If a core that leaves the production is a good one or not is kind of random. There is a margin.
This is how it probably works at intel:
After production those unpackaged cpu cores get tested and sorted in groups depending on the results of those test. There are those with good electrical values fully functional, those with not so good electrical values that are also fully functional ... and then there are those cores with good or not so good electrical values that have an error, like one broken cell in the cache.
The cores with the best electrical values that were also performing all tests without errors get labeled as Intels top speed models, except the market doesn't need so many top models, then they are just labeled as something cheaper and get a lower max multiplicator burnt in.
The cores that are functional, but have worse electrical test results get labeled as some slower E-6xxx and set lower max-multiplicator.
In addition to that we still have processors that fail the cache test. They can't be sold as E-6xxx core duos anymore. Besides the broken cache, the core can be electrically really good and support high speeds, but can also be bad, like I said before.
Instead of throwing all those cores into the trash bin, intel disables the broken cache bank and sells the processor as E-20x0 with 1MB cache and says it supports only FSB200 and a relatively low clock speed, which any of those cores supports with ease. Easy/efficient way for intel to supply the market with cheap low end CPUs. They don't have to run an extra production line.
And now the overclockers come, who are also happy about that and buy those CPUs. Depending on their luck they get processor with good or bad electrical values and high or low overclockability.
I've bought an E-2060 myself. The FSB is quite overclockable, but the clock speed of that processor is the limiting factor. FSB can run at 333+ without problems, no voltage adjustment needed, but the core starts to get instable and demands high voltage like 1.4V+, when it runs at 3Ghz already. So currently I run it with 2x2.7 Ghz to stay out of any trouble.
I'd say, I didn't have extreme luck with the core I bought, but I'm sure I didn't get the worst that's out there. If you buy a CPU for that price and it can be overclocked from 1.8Ghz to 2.7Ghz...3Ghz without getting hot, you can't really moan. Sure could have gotten a core that is capable to run at 3.3 Ghz by some luck, but I haven't.
edit:
But I agree to the other poster:
If I'd buy another one, I'd buy an E-2180 just to get the 10x multiplicator. Then it's still a matter of luck, if the CPU is electrically good enough to run stable at 3.3Ghz, no difference to the E-2140 and E-2160 core, but at least you have the 10x multiplicator in case it does and don't need to overclock the FSB beyond 330MHz.
