1. CPU: Depends what you're going to use it for... if you're splashing out and like to game, go for an AMD 64; if you're into hardcore video/music encoding then a Pentium 4 EE might be better for you. If you're on a budget, go for a regular AMD Athlon; if you can afford a little more, go for a P4
2. HDD: Anything big and SATA will be fine, from the major brands (Western Digital, Maxtor, Seagate)
3. 5.1 is great, but I don't know if 7.1 is any better.