If that sound card (or rather onboard audio) was good enough then I'll write that any model of the Creative Labs cards will be equal (I'm fibbing) or exceed (that's the truth) than the onboard audio. These Sound Blaster cards run from 29 to 199 bucks but even the SB16 PCI card has delivered great sound for me.

I'm sure that some ears are better than mine so you could consider the Audigy series for less than 99 bucks.

As to the PC, any current machine will outpace your current machine so that's just a matter of budget.