Can the duo core 2.4 GHz be OC to 2.66? How high can it be OC?
The limit is only your cooling and the amount of voltage you can provide the chip. A E6600, regardless of all but the worst batch which would most likely be sold as something lower, can reach 2.8 - 3.0GGHz with little to no change in the VCore or any other settings. And on the stock heatsink that came with it no less. 3.2 - 3.6GHz is within range although going above 3.4GHz requires both a good batch chip and if you want to preserve the working life of that Core 2 Duo, a better heatsink. Now 3.7 - 4.0 is still possible on air, but at those frequencies and the temps that come with it, you want to invest in water. Or a TEC. Or phase change.
Now if you're talking about a AMD chip, I assume its the 4600+ X2. That too can easily do 2.6GHz and with some good tweaking to the BIOS, 2.8GHz. Going beyond that usually requires better cooling though, but the 3.0GHz mark is still attainable.