If I were you, I would try and keep it under 65ºC load. I have read that the Q9400 can get up to 4.0GHz, but you would need better airflow/ cooling in your case to be able to safely reach that. A 300MHz Overclock isn't great, but it's a start, if you can, get a fan for intake directly over your CPU and Heatsink+ Fan. As always, when Overclocking, it is suggested to get an Aftermarket Heatsink and fan, but if you have good airflow in your computer case, there will be no need, but since you are idling around 55ºC+, I would try and get better airflow or a new heatsink. I'm not much into Intels, but I do, do a lot of reading, which helps, quite a bit in gathering up information to answers questions.
If you are anything like me, I like to overclock, but keep the Stock Voltages; which keeps me in the safe zone of not accidentally frying the CPU and possibly taking out the Motherboard with it.
For the best results, get Prime95 and run it now to make sure that you don't Overheat or have gone too far for your Motherboard. If you get a Blue Screen, then go back about 5MHz until you are stable on Prime95. Once you are stable or seem stable, leave Prime95 running overnight; make sure that you do the Blended Test which stresses your CPU and RAM.
One more thing, when you are adding fans in your computer case, take into consideration of the amount of air they are capable of moving, if they have a low CFM, don't get them, try to stay above 30-40CFM.
I am arranging a cooling solution at my earliest, would be nice of you if u recommend some, by type , like water cooled, ... or which one would be better in high ambient temps.
stock cooling which is not quite heavy in weight, mobo stand vertical coolers would stay horizontol. cooler would stand in those same four holes, would it stress mobo due to its weight etc or should it be mounted by some foundations underneath mobo.
How to fell and decide about other chips temps like NB etc
for CPU voltage ( currently auto in BIOS) I will visit intel for my CPU Q6400 and set that value