1) I'd go with AMD on this. Use a Sempron processor, but buy a motherboard that can be upgraded to an AthlonXP.
2) For motherboard, pick your other components and use newegg to select a model that fits your requirements. Then check the user ratings on that board.
4) I would suggest good quality ram with a good amount- think balance.
5) No. Use an AGP card- you can get a livable one for $45 from Newegg. Once again, just input your parameters and it will make suggestions. Newegg.com!
6) Forget the 6SATA drives; go for 4, which is standard in many mobo's. That way you won't have to pay an arm and a leg for something special.