Freenas 8.2 hardware help

[killbill]

Hello all

I will install Freenas 8.2 and use it as a file server to an OFFICE network with 15 pc with 3-4Gb data each. So my questions..

1) I will use a big hard disk for data and for the system (freenas 8.2) should i use a usb stick(8gb) or an ATA100 IDE 30gb hard disk which i really dont need it? which is faster?

2) If i buy 2 sata hard disks as RAID am i gonna have better speed thru my ethernet 100bps network?

3) My computer has E5400 intel processor and 2GB Ram DDR2. I will buy 4gb ram more or 6gb ram more?

thanks!