To answer that question, my experience has been that the more spindles you can involve, the better the performance. A lot of SAN folks will argue against that but I'm not sure why because it actually does make a lot of sense.

To wit, I normally try to separate MDF and LDF files to their own physical sets of spindles as well as setting up TempDB on it's own set of spindles. If I can, I'll set it up so that the MDF/NDF files of TempDB are on separate spindles from the LDF files but, no matter what, I try to put TempDB on it's own drive(s) so I can configure it differently than all of the others.

Of course and as a sidebar, that's not always possible with the ridiculously sized hard disks they have now. It was so much easier to right-size and get more spindles involved when disk size was much smaller. For example, I just can't see dedicating a 300GB drive to the LDF files of a system that won't grow to more than 600GB across multiple databases. It was a little tougher on electricity and cooling but it even allowed for faster disk replacement if one went bad because the system didn't have to rebuild so much.