Sound card first. It depends, onboard sound is a hell of a lot better than it used to be, I'm happy enough with it but it really depends on what you think. I would try it with the speakers and see if you're happy with it, if not then get a sound card.
Changing a processor, you can do it yourself, its not as easy as putting in more memory but it certainly isn't all that differcult. Before going into this find out if your motherboard can take a faster processor. If it can't then saying any more is a total waste of time. Download this and run it and see what it sayd your motherboard is click here then post back