System Setup suggestions

Hello,

I’m after some advice please.  I need to spec out a system for some software.

The software itself is fairly light weight.  It delivers the service via IIS (server 2008 R2) Along side it needs to connect to a MS SQL database and lastly there needs a file store area.

At the moment these three elements sit on the same physical server.  It has worked okay, but this doesn’t strike me as being the most resilient setup.

The new policy is to use virtual machines.   Given these high level requirement do you think a single server is the best option, or would it be better to maybe have two servers.  One for the app + files and another as a database server?

Presumably at the very least the drive should partition to separate OS from file store?
Appreciate any suggestions as to what the best setup should be.