<< You could always read this FAQ for another opinion, but I don't really agree with every point raised in this article. >>
The FAQ article is one of those mis-information on the net and unfortunately from Anandtech this time. People who works in IS support will advise you to take the recommedation from Mitzi as a mininum.
The OS, applications and games are easily recoverable from the original CD. Your personal data, no matter how meticulate you backup, is not. Things that can potentially damage the setup includes new program install, OS changes, virus attack, or hardware update. It is prudent to put the data folders in a non-OS partition. In case of emergency, you can always delete the OS partition, re-install the OS and recover without damaging your personal data.
I have a 24GB 4mm tape drive for backup and does regularly. I use 5 partitions. One for OS and utility programs, one for office application, two for data and documents. The 5th partition is for copying CD to HD to eliminate the need to grab the CD for some applications or games. It may not make a big difference in organization, but it sure makes it a lot easier for recovery in time of disk corruption. (Don't tell me you never had one. I use only Ultra160 SCSI drives.)
Another advantage of multiple partition is efficiency. The content of the OS and applications/games does not change rapidly. These partitions do not need frequent defragmentation. The data and documents change everyday, with old data deleted and new ones added. These partitions need frequent defragmentation. It is much more effective to defragment a smaller partition and keep the disk performance at tip top condition. We often schedule defragmentation to be done overnight unattended. The smaller partition facilitate different schedule for each according to its need. It also minimize the danger of damage if power interruption occurs due to sudden thunderstorm in a hot summer night.
The user should determine how many partitions does he need. Generally, 2 to 5 will be the right range for most users.