Well, you don't HAVE to buy a sound card. You won't have any issues with onboard, or anything like that.
Its just, if you've spent loads on speakers, you want the best sound possible, right?
As for AMD vs Intel, I thought Intel were cheaper at the moment, and more overclockable?
If you bought an Intel E6600, or something, which is dual core, I think it'd be pretty amazing.
They just seem better right now.
My old PC is running off a 3500+ though which is still functioning well, as I'd expect.