The NIC doesn't matter at all (absolutely no difference), and IMO the sound card doesn't either. There are some that would argue that you can hear a difference on high-end audio equipment (do you have any?), but I haven't found anyone willing to do a blind test.
You don't need SLI yet, but you'll be able to add another 7970 in the future when one is no longer enough. The single card should max all current games on 1920x1080. By the time you want another one, the price should have gone down a little.
Don't bother spending extra on the Caviar Black. Your system drive will be an SSD, which is worlds faster than any hard drive, and the only files on your mechanical hard drive will be ones for which speed is not important (music, documents, etc).
Why Do You Insist On Capitalizing All Of Your Words?