If your PSU is a good quality brand, it should be OK. If its a cheap no-name then it may be struggling. Have you plugged all required power leads into card? Is the colling fan on the card running? Install Speedfan, it will show temp of GPU and also let you monitor the 12V rail. If it drops under load you have a PSU problem.
Your PSU is at the bare minimum for PSU requirements on that card (450watt/24amp), and I'm not sure what they do for calculations but you might want to look at an upgrade there. While typically a PSU doesn't cause artifacting, it's possible. If you get a new PSU, then see if the card keeps artifacting. If so, then the card has some issues.
Sometimes cards only show their weaknesses in certain games/apps. I had a card that would work perfectly fine, except that some of the doors in Oblivion didn't render correctly and in the Deep Freeze benchmark, it would flicker a little bit. It was a video card problem. I sold the card to my friend and he has the same issues.