Depending on your I/O needs too, SQL Server can be a bit of a resource hog. If you have SQL Server 2008, you can use Resource Governor though to help out with any issues on that end. A 25 GB database isn't that big, but you also need to consider its growth rate too. It depends on your hardware set up also as I don't know how big your drives are or how fast the boxes are.
Good points about security too as SysAdmins, Network Admins, etc. are going to need access to the box. If it's sensitive information (e.g. HR database), then that should be on its own box regardless as the less people with access to the box in any way, shape, or form, the better.