I would recommend an abit 3rd eye, combined with some Kingston HyperX and a 3200+ Winchester core. Throw in some water cooling and you can overclock that bad boy up to a 3800+, possibly even higher.
Stick with the 6600GT, best bang for your buck, as well as onboard sound. Those PCI cards are not that much better than onboard, and their price just doesn't justify them.
Don't skimp out on the PSU or the case if you're going to be putting high end parts in, both are pretty important, especially if you are using a lot of power eating, heat dissipating parts.
Intel C2D E6320 / AMD Athlon X2 3800+
Gigabyte 965P DS3 / DFI nF4 Ultra-D
2GB OCZ Gold PC2-6400 / 2GB OCZ Gold PC4000
eVGA 8800GTS 320MB / eVGA 6800GS 256MB
150GB Raptor / 74GB Raptor
2x500GB / 320GB
OCZ GameXStreme 850w / OCZ StealthXStream 600w