500W (or 500ish for those manufacturers that dont make exact), actually the safe number is around 430-450 but factoring in possible future upgrades and the effects of capacitor aging. as well as the PSU efficiency.Doesn’t hurt for a bit extra:D

There’s no need for you to buy a 600W PSU unless you’re running dual NVidia cards and overclocking everything. Visit this page and key in the details to see what PSU wattage you need:

http://www.extreme.outervision.com/psucalculatorlite.jsp

Thanks for your advice, guys. Will look into it when I have time!

But you don’t necessarily need to buy a Corsair PSU,though they are definitely good. Just buy a name brand PSU. I’ve used AC-Bell(Taiwanese OEM for a lot of the bigger brands), Thermaltake, Enermax and Cooler Master in situations where they are all under prolonged high load and they stood up fine.

Name brands aside, what you need is a power supply that can give a steady output at its rated power, i.e the rating is the true power not the peak power. Often cheapo power supplies will say 600W but will only be able to sustain it for the whole of 3 seconds before they explode or crash.The reason for buying a good name brand power supply is that these manufacturers usually put the actual sustained output on the packaging instead of the peak power output.

For your needs (unless you have an insane number of HDDs and or a monster video card) I`m guessing 450-500w range would be more than sufficient and provide you with the option to add more drives in the future without worrying about power.

Expect to pay around RM300-RM400 for a decent 500Wish PSU.
When it comes to power supplies:

1)Don’t pay less, you will get crap that will do weird stuff like what you’re experiencing now.
2)For the same power rating, the heavier the better(weird in this day and age of ultrathins and superlights, but with PSUs this generally holds true)
3)A better power supply will better cope with the weird ass power fluctuations our friends at TNB get up to all the time , especially if its a house with older wiring.

That aside, there is a very mundane cause of this, which is rare but i`ve seen it happen way to many times(in production servers which crashed ONLY under load causing huge headaches) , open the machine up and check if the motherboard takes axillary power connectors. A lot of newer motherboards take 1 or even 2 aux power connectors in addition to the standard ATX one. Person that built your PC might have forgot to plug one in and that would cause EXACTLY the symptoms you described. Same goes for aux power on the video card (though this is even more unlikely)

If you do need to change your PSU, I recommend you buy Corsair PSU. They’re quiet and got enough power.