I am gonna take the unpopular route here and 'advise' you you should replace that power supply. No matter what.
Spending money on any decent graphics card while this power supply is in play is 'risky' , and I just can t condone it. It's your money and choice but there is a strong chance you could lose the entire pc.
http://www.tomshardware.co.uk/forum/id-2547993/psu-tier-list.html
The 500 watt unit is not listed. I would not trust this unit as part of my equipment,being honest.Best to budget for a replacement tier 2 unit.