Would it be worth getting another 2gb ram for dual channeling, or won't it increase performance much?
Will my CPU and GPU work together for DX-11, or will there be stuttering in games?
Does my PSU have enough power for my system at peak performance?
getting 4gb of ram would be a good upgrade but u have to check that the extra stick has the same timings, voltages etc. otherwise u might run into problems. 5400rpm HDD is also very slow, you'll see a performance boost if u get even just a 7200rpm HDD like the Samsung Spinpoint F3 or a WD Caviar Black.
100% support getting an SSD.
Are all modules of your 6gb ram identical? I'm not a fan of mixing and matching. In your system 4gb is more than enough for everyday usage, so going from 6-8 wont make any difference.