40 GB would be a WASTE of 30 GB if you managed the server properly. Almost everything can and should be moved off the C: drive. When you do that, your C: drive grows primarily due to new patches and their related uninstalls. On an SBS Standard box, I make the partition 12-16 GB (I make it 16, but I don't bother changing anything if it's at least 12 - most Dell's ship with 12 - or have shipped with 12). Otherwise, look at what the system is going to be doing and what it's going to use. Most Windows installs after years are only 4-6 GB. Then the program files should RARELY grow - this is because this is a server - you shouldn't be installing a lot of things on the system = only stuff necessary for your server to perform it's duties. It should be very RARE that your Program Files folder is larger than your Windows folder after 1 year. Likewise, Documents and Settings should be small, each profile under 250K and no more than 2 or 3 administrator profiles.
Even a non-SBS server can use this little - or less - they are doing less and need less installed.
The one server type where I'd do an entire physical disk as the C: drive is a Terminal Server - that you might install a lot on - and you'll have (presumably) lots of users who DO NOT keep their profiles small, so on Terminal Servers, lots of space is NOT a waste.
Now, space is cheap, relatively speaking... but hate wasting 10 GB of space - or more. I want consistently 3-6 GB of free space, but after that, I'm not worried and none of the SBS servers I've managed has had an issue.