Hello Adrian, I am sorry to hear you are having problems with your system but let me help you.
Up to 256 MB of system memory can be allocated to DVMT on systems that have 512 MB or more of total system memory installed. DVMT returns system memory back to the operating system when the additional system memory is no longer required by the graphics subsystem.
So, when you set “maximum DVMT" in BIOS it is going to use 256 MB of video memory for integrated video card. In your case, since you already made all this changes and you are using the latest BIOS version available, what I can recommend you is to roll back to the previous BIOS v70 using the recovery method. Then, check the settings again.
Additional information Graphics — Graphics Memory FAQ For Windows 2000/XP/Vista/7*