I plan on putting none of the system partitions on the RAID5. Everything system related will be on the 80GB drive, including tmp and swap, but there won't be much of it, maybe 1GB. I don't like the idea of swap on RAID5, but plan on having it on the 80GB for when I get a second drive and mirror.
I keep /tmp separated due to the fact that I dont want a full /tmp to interfere with the root volume. It is how I have always done it and see no need to change to something that may be less resilient, even if it is slightly more convenient.
Also, I believe that the OS can cache the requests until such time as they can get written, and then do a fsync.
If the host controller supports it, the single channel should not have a problem:
FIS(Frame Information Structure)-based switching
FIS-based switching is similar to a USB hub. In this method of switching the host controller can issue commands to send and receive data from any drive at any time. A balancing algorithm ensures a fair allocation of available bandwidth to each drive. FIS-based switching allows the aggregated saturation of the host link and does not interfere with NCQ.
from:
Port multiplier - Wikipedia, the free encyclopedia
It turns out, the controller that ships with my enclosure, the Sil3132 supports FIS.