overclocking doesn't burn stuff. if its cooled well enough to run at stock speeds its cooled well enough to run at its maximum overclock. if you get artifacts (visual errors displayed through your monitor) then its likely you overclocked too much and its unstable. that just means it calculates wrong and you need to take the clock speed down a bit.
anyways, overclocking an 8400gs won't really help your performance a whole lot. you see video cards don't run the way cpu's do, more like how multi-core cpu's do in a way. say you have a 7800gtx. they have 24 pixel pipelines. well my old 7800gt only had 20 pixel pipelines. the problem with overclocking my 7800gt is that it only got 20/24 of the performance that the 7800gtx did per clock.
basically, your card has few processors, not many processors like a high end card, and therefore increasing its clockspeed even by a lot wont make it perform marginally better.