I did some calculations on your PSU and after derating and apply the design standard, it would be lucky if that PSU could provide 13A to your graphics card (160W approx). Not enough for the 12V rail minus the CPU. Remember thats when it shuts down too (assuming it can provide what it says on the tin which it probably cannot).
The CX600 is a good choice. Im glad i pushed you to look into that, it couldve been nasty
I'd get this
TX650 by corsair which is $70 after rebate etc.
And this
GTX570 which will smash both of the above
All for about $340. Great step up for both quality and power.
If you wanted to keep it closer to $300, then the
CX500 is $50, but you will have no head room for upgrades like SLI (you will with the TX650).