In it, it says that 3D testing is also a very important one that many people miss and cpu that passes 24 hour prime95 may fail 3Dmark test quickly.
"When it comes to stability, 3D testing is often overlooked completely. Many people do not overclock their video cards, and as such decide that 3D stability tests are a waste of their time. They couldn’t be more wrong! An increasingly common phenomenon is an overclock which will pass Prime95 and memtest86 for 24 hours, but lock or crash 3DMark in a few minutes – it is for this reason that 3DMark testing is a good idea. And for those who do overclock their video cards, 3DMark is an invaluable video card stability testing tool."
Is that still the case for modern chips like sandy bridge?
Also, how many instances of prime95 do I run simultaneously on i5-2500k chip to get accurate result?
If you have the newest version of Prime95 (26.6 I think), then you only need to run one instance. Just go to the Options menu and choose Torture Test. In the window that comes up, make sure the "Number of torture test threads to run" option is set to 4. It will then load all four cores to the max.
And yes, you should run some sort of graphics stability tester as well as the games you normally run. I use Afterburner to overclock my GPU, and Kombustor for stability testing. I also use FurMark, but some people avoid it because of the chance that it can kill your graphics card.