Most, if not all, motherboards come with onboard sound, so you don't
need an addin card, though you may be able to tell the difference in quality.
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Either way you go, you can't go wrong, whether dual core or quad.
Personally, I went quad with a guaranteed G0 stepping (revision of the CPU generally easier to overclock). Even if I don't overclock, i'll be happy with the purchase.
That's because I do more than just game. I do a lot of video conversion to my PSP and home movie encoding, etc, so the more cores the better (for the most part).
And on the horizon, I think games are going to become quad-core aware, especially since they're getting cheap now. I know Crysis can take advantage of 4core, and I think Supreme Commander also has a great scaling with cores.
I seriously hate referring to the Xbit labs article on the Quad vs Dual because of the total mismatching of the cores...Why the hell didn't they compare it vs. a E6600 I have no idea, see as they are the same clock speeds.
For a more thorough comparison, check out Tom's Hardware CPU charts.
http://www23.tomshardware.com/cpu_2007.html
Select the 2 CPUs to compare and the application and it highlights those 2 you selected
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Either CPU you get is certainly not a bad choice, and for right now your dual core will be better for gaming, but quad core is also an excellent choice.