For overclocking it usually makes more sense to go with the lower standard clocked chip as they will probably overclock to the same level, and it will be the cheaper option. As the i3 chips aren't out yet its hard to tell, but if you compare the i7-920 and i7-940, overclocked results will be similar and the 920 is about half the price.
Not sure about the i3s but there could be a difference in how far each chip can Turbo, assuming it can even Turbo. (Haven't looked in to the i3s yet and there aren't much reliable info).
a) I believe its fairly well established that the i3 will not have the turbo (Clarkdale)
b) My question was supposed to be generalised with the Core i3 as an example only. This why the title was 'different chips in one family' (whatever that family is)
c) My key question was
If someone buys an i3-530 and clocks it up by 133mHz, do they effectively have an i3-540?
What is the difference between this and buying an i3-540?
For i3-5xx sub in anything else you like. Refer to the E7XXX or E8xxx series of chips if you want. My question is about the nature of overclocking I suppose, as compared with what the manufacturers do with the clock multiplier to differentiate their models.
This was really meant as a general question, not one about any specific product (unless the answer happens to depend on the product).