Linux as a NAS performance questions

[Neil Joseph Schelly]

I'm looking to build a small Shuttle barebone machine into a NAS running 
Linux.  The intent of the machine is to be a networked PC with lots of 
storage in a RAID array, made available over the gigabit network interface 
via Samba, NFS, and maybe iSCSI protocols.  I'm curious what experience 
others have with this sort of stuff in general, but two immediate questions 
come to mind about processor and memory performance.

I can go the low-power, low-heat route and get a single-core processor and a 
single memory stick of minimal quantity.  Or I can upgrade a bit, get a 
dual-core processor with 2 sticks of dual-channel memory.  Or something in 
between.  What I don't know is how much impact processor speed, multiple 
cores, memory capacity, and dual-channel memory has on disk I/O, network I/O, 
software RAID processing, etc.

I like the idea of a small low-power, low-heat appliance, but will going too 
low on those negatively impact performance much?  The cost difference between 
a single-core processor with 1GB of memory and a dual-core processor with 2 
sticks of 1GB dual-channel memory is insignificant, so that's not much of a 
concern.
-N