I bought a Sapphire HD 6670 ,it worked fine for about 10 days but while playing crysis my screen started flickering brightness ,my screen brightness continuously goes low or high.When I consult it from the computer dealer he told me to change my psu which was only 180 w and i replaced it to Corsair cx500 but the problem is still continue.My game also get freeze while playing ,it worked fine when i connect it to onboard video card.I have also checked to adjust refresh rate and reinstalling the graphics.
Minimum spec's have _nothing_ to do with the problem he's describing... The Power Supply could of affected it, but you've ruled that out with the CX500.
Shaswat, if you haven't sorted it by now I'd recommend testing with an alternate Monitor & Monitor cable. Try alternate graphics drivers as well, if problem still exists your graphics card is most likely faulty. If after testing or trying with another graphics card it's still doing the same thing, then I'd be looking at the Mainboard.
As MajinCry was saying with the RAM, you do have a very under spec'd computer for running Crysis, given the price of RAM/Memory these days, put as much into your computer that you can afford will make quite a large difference.