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.