You don't need extra partitions in modern file systems. You had too before because as partition size went up, so did the cluster size (minnimum file size). So eventually you would get to the point where a 1 byte long file took up 100 K (about 100,000 bytes). That was a waste of space.
If your using windows XP (or 2000) NTFS file system extra partitions are unnessesary. If you really want to divide out the space there is a variety of tools to simulate extra hard drives (dos command subst) or enforce restrictions on folder sizes.
If you went extra partitions your going to be kicking yourself when your GPS partition has 100 mb free and you just need another 5 mb for a song on your mp3 partition. Yes, you can resize them with partition magic but thats always risky.
The only practical reason I can see for multiple partitions is multiple operating systems (most linux distros will need an ext3 or reiserfs partition and a swap partition) or for specialized compatability issues (windows 98 cant read NTFS, in this case you would want a FAT32 file system as well)