6 GB of RAM installed, but only 2GB shows up. How do I change this?

Hello, Dood here.

I have a ~3/4 year old Aspire M3470. Stock motherboard is daa75l aParker and is as shown in the manual. I have three 2GB RAM sticks (two in black and one in the middle). The BiOS menu confirms that there are 6GB installed, as does the "Performance Monitor" inside windows. What the BiOS menu doesn't say, however, is that 4 of that 6 GB is "Hardware Reserved," which to my knowlege means it is being used by the BiOS to carry out tasks. If the problem is the BiOS, a replacement might be necessary, as there are no more BiOS updates for this motherboard. Nothing in the "Boot" menu in advanced settings for msconfig is checked (maximum memory, etc.) and when I take out one stick of RAM only 2 of the 4GB of RAM is "hardware reserved," so no matter what I only have 2GB of "usable" RAM at one time. As you can probably imagine, this hampers the overall speed of my computer. It is almost certainly nothing else, as my APU (stock, AMD 8-3800 2.4 GHz) usage is nears 80% when doing hardcore rendering. GPU is NOT stock, it's a MSI GTX 760 put into the PCIe port in the motherboard. That itself has 4GB of VRAM, so stress is taken off my computer somewhat. Power supply is 620V (Seasonic SS-620GB Active PFC F3 is what it says on the label). I was wondering if this was a quick fix, a not so quick fix, or I should just replace the motherboard (with possibly the CPU as well, so costing a pretty penny).

Don't be afraid to ask if there is anything unclear.

Thanks, Dood