Dario43,
On behalf of Tom's Moderator Team, welcome aboard!
You didn't quite answer the question, which was asked twice:
Do you have Windows 64 bit or 32 bit?
It matters because Windows 32 bit is limited to addressing only 4GB, and as such is not able to address 8GB or more. In order for Windows to address all 8GB, you
must have Windows 64 bit.
Your AMD Sempron 145 and Asus M5A78L-M LX motherboard both support up to 8GB.
Once again, welcome aboard!
CT