For a system using a single GeForce GTX 280 graphics card NVIDIA specifies a minimum of a 550 Watt or greater power supply that has a combined +12 Volt continuous current rating of 40 Amps or greater and that has at least one 6-pin and one 8-pin PCI Express supplementary power connectors.
Your Nexgen 600W PSU, with its +12 Volt continuous current rating of 25 Amps, is grossly inadequate to power a system with a single GeForce GTX 280 graphics card.
A reputable name brand power 550 Watt supply will have at least a +12 Volt continuous current rating of 45 Amps.
Get a power supply from reputable brand names like Antec, Corsair, Enermax, Seasonic, XFX.
I'm not so sure about the defective cards post... I'd just like to say those 280 gtx's are a very underated card. I personally have had one since they came out quite a few years ago, mine is 280 GTX OC edition by the now extinct BFG Tech. I now use it in one of my folding @ home computers. Here's why I feel (or at least mine) is a god card: stock gpu clock was 615mhz I have run mine in my folding @ home farm for 3 days straight AT 740mhz, you would think this would fry the card? NOPE! Never even got above 75C. I have been running that computer for those 3 day cycles forrr about a year now, have had two HDD's go out but the graphic's card is still running strong.