well - I don't ever consider the motherboard soundcard to be sufficient, not for real recording. I have recommendations on my website (in my sig) for internal and external soundcards. there are no USB3 soundcards yet, USB2 is sufficient. there are no eSATA soundcards either. there are PCI and PCIe soundcards and Firewire available. if you get a firewire card, I suggest getting a 3rd party PCI or PCIe firewire card with a TI chipset (that's what I run).
as far as the rest of your computer, it's more than sufficient.
SHS6 ahas both 32 and 64bit installations, but the bitbridge for 64bit version for SHS6 wasn't that great from problems I've read on the forum.
I'd advise installing both versions (you can install both and run either one). run the 64bit one if it doesn't give you any problems because it will allow you to use more memory with SHS, use the 32bit version if you have problems with the plugins or softsynths in the 64bit version.