It depends on the quality of your 500w psu. If it's generic or no name then I would not use it 'cause most no name psu's are not really good. Some are rated for 500w and can't even put out 350w in real life situations. Remember that if you stress your psu too much it can fry and take out other components with it like ram, cpu, mobo, video card ... do you think it's worth the risk ?
That's a nice rig you got so I would not skimp on the psu, as others said Corsair is a good brand and so is PC Power & Cooling, Antec is also good as are the new psu's from BFG.
I've read that article already and I think they are right .... but I think it is better to have more power (30 to 40%) than needed so your psu will not be running at full capacity all the time. According to the article you linked , with my system I would need between 350 and 400w so I know that with a 650w psu I will be fine.