lol, I think you missed what I was driving at. I'd sooner have the OS away from the data drives entirely. For a DC I'd use RAID-1 for the OS. For a non-DC, reinstalling an OS on to a blank HDD is no big deal since the data is safely tucked away on the seperate RAID-5 volume.
Like I say, using a single RAID-5 volume and partitioning for OS and Data may effect access speads.
I'd agree, which is why I originally wrote:
RAID 1 for OS, RAID 5 for data or RAID 5 for OS and data (partitioned) is how I do it.
Three different options, but it all comes down to cost/size of the network too as you also pointed out.
I can confirm I've noticed no difference in speeds using a single partitioned RAID5 array. DCs really are not that CPU or memory intensive - plus I've looked at loads of servers (not setup by me) setup in the same/similar fashion. Custom builds and big OEMs.
I suppose if speed was an issue, SAS rather than SATA would be the next step.