Other people here may recommend particular answers to those questions.

All I have to offer is, Linux has *blazingly fast* block I/O, and the kernel page cache does a super (and transparent) job of caching data. Give the machine as much RAM as you are comfortable (budget, power) with, and don't worry much about where the data resides. So long as it's on ATA, SATA, or GigE, the pipe is plenty fat enough. Even USB2 (~35MB/sec) is probably quick enough.

Cheers