IMHO, don't bother with manual placement since it's an inaccurate science..also the list of files that you use most frequently will change with time and it's not feasible to do manual placements frequently? Just go for a good auto defragger that checks the drive for the fastest areas and automatically places the most frequently used files there for maximum performance. It takes the guesswork out of which file to place where and when.
As for installing games to the OS drive, I see no problems..should work fine.