I think it's, fundamentally, a good idea to separate your PC's programs , swap & temporary space from storage used for media files, especially, anything used for video/DVD.
I've spent a long time working on UNIX systems and the philosophy of separating programs/swap/temporary/application data is standard practice although drive capacities and virtualisation makes it difficult to hold to an ideal.
Performance is one issue: if you've got more than one physical disk drive available then stripe for performance.
If you're buying new to expand an existing system go for S-ATA drive(s) and a new S-ATA interface card (Adaptec do one for about £50 that'll support mirroring or striping). Mirroring (aka RAID-1) will ensure that you don't lose data in the event of a disk fail and provide a small improvement in read performance while striping (RAID-0) will improve performance but if a disk fails make sure you've got a backup.
It's possible in Windows 2000/XP to concatenate physical drives to create a spanned partition as a new volume or expand an existing partition.
It's not, necessarily, a good idea to allocate drives/partitions exclusively for music/video/photos - I'd recommend single drive allocation (D:, E:, whatever) with a well organised folder hierarchy.
Do realise that if you split a physical drive into more than one partition it isn't going to have any bearing on performance - there's still only one underlying spindle/platter set to read and write data from/to.
Fragmentation is going to become an issue on larger volumes with large audio, photo or video files so defrag regularly.
