Firstly be careful with partition magic, if used incorrectly or even if you're just plain unlucky you can end up messing things up pretty badly.
I would recommend not messing with the boot/system partitions otherwise you can very easily find yourself with a system that won't boot and needs some creative repair.
Aside from that it doesn't matter a grat deal if you're using NTFS as its pretty efficient at all partition sizes so really its just a question of how you want to organise your stuff, the main advantage (other than better organisation
) would be for defragmenting i.e. if you keep your OS on one partition seperate from your other stuff you'll spend less time defraging as most fragments will occur in your OS (you shouldn't need to defrag the other partitions as much).