I went all "Hey BIOS I hate you and you hate me but I'll mess with you" on it and turned everything at NorthBridge configuration to either "Disable" or have chosen the lowest option and - voila - suddenly I have:
"Installed memory (RAM): 4,00 GB (2,99 GB usable)"
CPU-Z shows only 1 graphic card (the Radeon) so I guess I disabled the onboard CPU graphics. I understand that those 2,99 GB is the top number I can have because of the 1GB my graphics card has, right?