All I can say is that the RAM you have is not right for your computer. As myself and Absic said before your PSU is also an unknown factor in so much as it is underpowered.
As for the memory if money is a problem then I would p[robably go for this:
CMX6GX3M3A1333C9 DDR3-1333 (XMS3-10600C9*3CH) 6GB Kit (3 x 2GB)
If you can afford more then go for the Dominators
TR3X6G1600C8D DDR3-1600 (Dominator-12800C8*XMP*3CH) 6GB Kit (3 x 2GB)
In the end it has to be your choice. It would be very easy for me to sit here and spend your money. Of course it all depends on what is available in your region too.