i would usually do
C: 65 GB for OS (If its windows 7)
C: 45 GB for OS (If it is Win XP)
D: 100 GB for Programs (I usually install all the programs here, use custom installation wherever possible. I also change the default location from C: for MY Documents, My Pictures. Desktop, etc to here, so that in an event of a OS failure I can format C: and begin with a new OS install)
E: 100 GB for Game Installations
F: 100 GB for Dumps (Basically image files of Downloads, Games and OS'es)
G: Rest for Music&Videos (I create 2 Folder Music and Videos, sub categories them as Hindi, English and then again as Movies TV Misc, etc)
Also install a defrag software ASAP such as Diskeeper and likes. Also do chkdsk every few months.