What about Power supply (PSU)??
Does it give enough power for all your hardware??
By the way I will double check the RAM. Probably is no bad by I have seen that happenning more than once. Try this:
Open the case, and remove the RAM sticks, exchange the position bwn them and restart the computer.
As I said make sure you have a good power supply.
To run that video card they recommend a power supply at least with 250 Watts + the two ram sticks + HDD + Motherboard+ CD DVD Drives + anything els you have in your system...
Read the instructions. This is an example:
Motherboard 45Watts
HDD 30-40 watts
Microprocessor: 55-100 watts
CD-DVD Drive 15-25 watts
Video card - 150 watts
That will be a total of 360 watts. So you will need a power supply of 450 Watts in that case.
So DO THE MATH and see if your power supply is giving enough power to your hardware...
Let us know...