Please help me:

I had 2x 2GB sticks of Apacer RAM, than I wanted to upgrade to 2x 4GB Crucial. But after booting  I had problem with noise and lines all over my screen...  I tried also 2+4GB mode, everything works fine, but Everest benchmark has shown not as good results as in 2x2GB mode. What should I do, to make it work with 8 GB of RAM?

My specs are: Windows 8 Pro 64-bit, 263 BIOS, AMD Sempron 2000 MHz, 480 GB Kingston SSD, ATI Radeon Xpress 1150, latest possibe graphic card driver (10-2_legacy_vista32-64_dd_ccc)