So, you have one 4GB stick, and one 2GB stick?
You'd be best off going for 4+4; identical sticks of RAM let the memory controller operate in dual-channel mode (which is basically twice as fast as 'single channel mode', which it would operate in with either one stick, or non-matching sticks.
Beyond that, your system's pretty well-balanced, and any one upgrade may require a revamp of the whole system to see much benefit, but you could upgrade the graphics card if you wanted 🙂