BIOS it recognises the full 1512M but in the memory test at the startup, it only reads 256M.
What does this have to do with memory speed?
In the BIOS it recognises the full 1512M
I have never seen a BIOS that shows you the amount of RAM installed. This is not a user built PC, is it?
Have you tried disabeling the Quickboot option in the BIOS?
Is all the RAM detected by Windows?