Re: Memory - 2 slots or 4 slots?
2 sticks is better if you're going to overclock. If you're not going to overclock there isn't any difference unless you're putting enterprise level application loads on your pc.
Besides that, performance related to memory in today's machines really only comes into play under 2 circumstances:
1. You don't have enough memory.
2. You're moving huge amounts of data through the system i.e. video encoding
MSI MPower Z87 | 4770k | CNPS9900MAX-B | 8GB G.Skill 1866 | EVGA GTX760 |ThermalTake 850w | X-Fi Titanium | HAF | Asus PA248Q | Lycosa | Death Adder | Win8 Pro 64
MSI GD65 | 2500k | H50 | 8GB G.skill | GTX570 | Ubuntu 13.10
17" Inspiron | i5 | 4GB DDR3 | 500GB HDD | Win7 Pro 64