indicate a limitation of 4GB for that MB. Might check to see if that's a limitation of the Intel G31/Intel ICH7 chipset....and therefore your MB.
MB specs show:
"4 x240-pin DIMM, Max. 4 GB, DDR2 1066*/800/667 Non-ECC,Un-buffered Memory
Dual Channel memory architecture
*Support max. 2 DIMM modules only for yellow DIMMs when over-clock to DDR2-1066
*When installing four DDR2 DIMM modules, install only single-sided memory modules.
*When installing total memory of 4GB capacity or more, WindowsR 32-bit operation system may only recognize less than 3GB. Hence, a total installed memory of less than 3GB is recommended."
If you truly need a system supporting that much more RAM, you'll probably need to switch to a new HW platform.