4850 freezing and showing random green lines?

Hi,

My pc just freezes and I get these weird lines across my screen. This has happened with c&c tiberium wars, kane's wrath, trackmania nations forever, rainbow six vegas etc

I have downloaded the latest drivers...

I am using:
a 500w psu but its probably about 2 to 3 years old could
AMD Phenom II x4 940
Asus M3A78-EM motherboard
4GB Transcend aXeRam DDR2-1066
2 x Hard Drives - 320 and 500gb
2 x 80mm fans and 2 x 120mm fans
Dvd rom

Could the problem be that I don't have enough power? My gpu is about a week old and its been doing this since the start really.