Dual channel is a big deal. Your motherboard supports dual channel so all you need is two sticks of ram one in each channel to have dual channel. The ram I suggested will do that of course.
The sound card is great, much better than onboard. Onboard will sound like crap on good speakers like the ones you are getting. Get the sound card.
Yes, Dual Channel IS IMPORTANT. If you go with single channel ram you will CRIPPLE YOUR RIG. Dual Channel is a MUST, unless you don't care much for performance.