SQL server Installation
In short, put the OSes on slow storage and all the data/log &amp; backup files on fast storage.

I know that my environment is different that yours, but I think similar principles apply.  We have a SAN and the storage pools range in different speeds, 7.2k, 10k, 15k.

We install the OSes on the slower drive pools and have all backup files and data/log files stored on the 15k drives.  There was a time when we accidentally put everything on the 15k drives and we didn't notice a performance difference.  It was brought to our attention by our SAN admin after an audit that the OS was stored on the faster drives.

We monitor SQL Server performance with perfmon and Dell Spotlight.  For grins, we went and looked at the historical metrics that mattered, and saw nothing to note.