I'd stay away from Aberdeen if possible, just because they are so expensive. I've had a lot of success ordering from
www.mwave.com and
www.tcwo.com
I'd consider a larger RAID too as in more drives, plus a mirror (RAID 0+1) The striping will help greatly when people download from your system.
I haven't built a server for this kind of use, but I have to question the benefit of a quad board. Are you sure you'd really see a significant benefit from this. Seems to me like bandwith is your largest hurdle??.