Space is definitely the big concern especially with large indexes and archived mail folders being stored in the OS partition. You have a few things on your side with Windows 7 though. With M$ Vista and 7 they provide the ability to partition on the fly so if you find you are running out of space on one partition it is possible to increase the size to compensate. Just curious, what is your reasoning to move to two partitions?
We were running two partitions with Windows XP on about 500 desktops but are switching back as we upgrade to Windows 7. You will not see any benefit performance wise from using two partitions if they are both on the same physical disk.
