So what you are saying is that where windows under system tab in control panel it says 8gb. So I am thinking that you use some type of utility in windows to check to see how much ram is available; something like task manager? Or is it the Asus bois that only registers 3 something amount of memory. If it is in the bois then what you need to do is to try a different memory setting. Your memory is set too high or does not have enough voltage. It could also be a performance enhance feature in your bois for the the memory! Not to say you can't run with this feature on but for now try running with it off. Make slight adjustments to the memory voltage,stepping and so on. Benchmarking the system after each adjustment. Once you have respectable benchmark numbers then go back into bois enable performance feature at a low level(processor speed) with you new found memory voltage,stepping adjustments.
If it is task manager that says you only have 3. something of memory available that is due to the programs you are running in windows using almost half of you memory at that point.