That CPU was right around the time when AGP was being phased out for PCIe, and most of the PCIe MOBOs had a 2X2GB 4GB limit of for DDR2 and AGP 2X1GB 2GB for DDR. The CPU itself is limited to 4GB via it's 32-bit instruction set.
As rolli59 mentioned - Name + Model of the MOBO; see Mainboard tab.
However, your PC is 6~8 years old and it's really time to buy all new. The differences with the P55/P67/X58 are 4~6 times faster, and having 4 to 6 cores is ... well a joy.