No there is no such thing as a safe percentage to overclock or anything along those lines. More or less to keep it simple, as long as you are able to keep it in the recomended voltage/temp range under extreme conditions (stress testing, on a hot day in your house) than I would call it a good. That is considering if you can make it nice and stable during that process.
And just so you get an idea on how the whole GHZ thing comes into play in OC, a really simple version again.
FSB x Multiplier = speed (ghz)
Manufacturers usually lock the multiplier lower usually to decrease the speed so they can have multiple models, for the sake of argument lets say we have 2 processors in the same family, their the same processor but have different multipliers say one with a multiplier of 8 and one with a multiplier of 12 (e2140, e2220 multipliers I chosen specifically). Thier FSB is 200, so your going to have a speed of 1.6ghz and 2.4ghz. Now there are 2 ways from what I showed you above to make it faster increase the FSB or the Multiplier, since the multiplier is out of the question since its locked, we shall pump up the FSB. Lets say we managed to increase the FSB to 400 on the first one granting us a speed of 3.2ghz and keep it stable. Now we tried it with the other chip with the higher multiplier but it was unstable since it was clocked at 4.8ghz and we hit a wall where the chip wouldn't exceed, which may be a FSB/Multiplier/Heat/Voltage wall, it doesn't matter. So now we have to back down till we get it stable, and we may just get lucky and get it to be in the same range as the e2140 or higher or if were unlucky it actually be slower.
So it's a good idea to research what your buying and how it overclocks if that is your intention. But just so you know the family of chips you mentioned would OC to around 3-3.2ghz on average which most people find to be the sweet spot. I currently have my e2180 OC to 3ghz with a pretty low voltage and only brought up my temp slightly, and thats only because of a poor rear ventalation where the case blocks 50% of the airflow in the rear vent.
Though I also recomend looking into the e5200 which is a newer low end chip brought in by intel if you in the market for a new low end chip, its a newer chip based on the wolfendale's and you can clock them quite high. You probably be able to get 4ghz out of one of those if you really wanted, and since it has a 12.5 multiplier that part wouldnt be part of the problem. Though there is no telling how long it would last at such speed, you need to look up how long chips in that particular family have lasted so far to get an idea.
Oh and read the OC guide that is stickied on intel dual/quad cores, it contains a lot of useful info.
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