For how you are intending to use it, I think 1 partition is fine.
Most people can't do that though, so multiple partitions helps because:
* If your Windows gets hosed, you can clean reinstall Windows and leave all your media and other files untouched.
* More partitions keeps data "contained" to a portion of the drive physically. So fragmentation can't go putting parts of the files large distances away. This may have been more of a concern back in the 9x days than now, I'm not real sure anymore..
Those are the 2 major ones I can think of. Out of habit I still put programs on D partition, but I can't think of too much of a good reason to do that, although it does serve as a reminder what programs I had installed so I can get them reinstalled right away.
What you might consider while everything is still fresh, is dropping off say... 2 gigs off your main C. Then also doing that on your 2nd hd (maybe you don't have to, but it might make backups easier if both partitions are identical size). If you do this, then you can have a 2 gig swap partition on your second disk which will speed things up a bit. If you can do this, put that 2 gig as the first partition, and the large one after that.