What you can do with the second SBS server licence
You can check the official description of what you are allowed to do with it (http://go.microsoft.com/f... but in a nutshell, SBS 2008 is licensed for a single server, which may be physical or virtual. The additional Windows Server 2008 may also be physical or virtual, but if it is virtual you are also allowed to run a further physical instance. You also get the right to run any number of instances of SQL Server 2008 on one server.
Here are some possibilities.
Use the second server as a backup domain controller. This could be located at a remote office to speed performance for those users. You can make this a read-only domain controller, which is both more secure and more efficient since it reduces network traffic across the wide area connection. Note that you cannot install SQL Server 2008 on a read-only domain controller.
Use the second server to run Hyper-V, and run SBS 2008 in a virtual machine. The main advantage is that disaster recovery to new hardware is simplified. Since Hyper-V virtualizes the hardware, you can in effect restore to the same machine. You are allowed to run a second Hyper-V instance of Windows Server 2008 for SQL Server or other purposes. Microsoft recommends that in this scenario you do not join the host machine to the SBS domain.
A Hyper-V host benefits from generous RAM and a quad-core (or more) processor. Microsoft recommends at least 6–8 GB for SBS 2008 (4GB is a bare minimum). There should be 2GB free for the host, when the guests are running. Overall, 16GB or more for the host would be comfortable. Note the backup issues mentioned above.
If you are using Hyper-V, it is worth fitting a second network card dedicated to SBS 2008, so you can use the first one just for managing the host. Configure the Virtual Hard Drive as fixed rather than dynamic, since performance is substantially better; you can still expand the drive manually if needed. You should not pause or save state for an SBS 2008 virtual machine. In the Hyper-V settings for that machine, find the Automatic Stop Action under management, and set it to shut down rather than save state. You can also set the Automatic Start Action to start automatically, so that if the machine reboots after applying patches or suffering a power failure SBS will come back up.
Use the second server to run ISA Server 2006, or the forthcoming Forefront TMG (Threat Management Gateway), which is the successor to ISA, now in beta (www.microsoft.com/downl...). This is a way to recreate something like the old SBS Premium edition which included ISA, although on two servers rather than one. Note that ISA 2006 does not run on Server 2008, so for this you need to exercise the second server downgrade rights to Windows Server 2003.
Use the second server to run 32-bit Server 2008. This is a good solution if you need to run a server application that will not run on 64-bit Windows.
what not to do
The second server cannot be used independently from the SBS 2008 server, for example to run a second domain. Nor should you run the second server within Hyper-V running on SBS 2008. Ideally hosts should only run Hyper-V, whereas SBS is already overflowing with installed applications.