My answers:
1. The info store over time will definitely far outstrip the size of the log files, unless you don't properly maintain your system. You can limit the growth of the log files by doing proper on-line backups, and this is very important both for controlling storage needs and making sure that your exchange info store can be recovered properly if a disaster occurs. The size of the info store, however, greatly depends on the size of your organization and the types of emails you receive (i.e., whether they contain graphics, lots of attachments, etc.). 10-15GB is not an unusual size for the info store of a medium-size organization.
2. My preference would be to have the log files on a separate volume from either the system orthe info store. However, if you don't have that option, I would recommend putting the OS on the same volume with the logs rather than the info store.
That said, I run Exchange in very small organizations (50-100 users, info store size between 2GB and 5GB). I have put the transaction logs on the same physical volume as the info store and have never had any problems with that. I really don't think it makes that much of a difference in performance in this small an organization. I usually dotwo backups a day - one at night and one at noon, just to make sure that my transaction logs don't build up and that I can restore both the full DB or any individual mailbox with minimal loss of data.
Hope this helps!
