Overclocking is an interesting debate and you can find plenty of info on the web that discuss the art.
My personal opinion is that as long as you do it within reason, and leave enough buffer room for just in cases, it should be harmless. If the reported MAX by ATITool is 412/245, then I'd clock my card at least 5% below max just to be safe. Hence why I am running my card at 390/230.
The truth of the matter is that chips are pretty much the same when they roll out the factory. Take for instance, a P4 1.6 and a P4 1.8. Architecturally speaking they are identical, with the only difference being speed. The speed is determined by series of tests on the chip's tolerance. Then Intel proceeds to lock the multipliers.
The idea is that a chip with greater tolerance at higher speeds would be branded as a 1.8, as opposed to 1.6. Technically, they are the same chip.
Hence sometimes you can get a 1.6 that has enough tolerance to run at 1.8, and in that case it doesn't hurt to run it at 1.8 because it is basically the same chip.
There's an Anandtech article on M10 classifications, having M10 w/at least 64mb and core at 333 makes the chip a Pro, which means that the F2 and G1 are at least M10P. The Pro Turbo is at 400/240 w/128mb. technically, same chips but the vendors can choose where they want to brand the M10, at pro or pro turbo etc... Basically if you've got a pro, there's a good chance it will run at pro turbo speeds with no problem.
In my experience, MEM speed is the bottleneck of achieving higher speeds. ATI cores generally have good tolerance, but their MEM speeds are finicky.