Hi Wolfgang,
Backup Exec 12.5 will support an agent installation on your Win2K8 R2 server.
In this case, it will do, but going forward, there isn't much mention of support for applications like Exchange 2010 etc.
So keep BEWS 2010 at the back of your mind for upgrade purposes as, and when you can do that.
My suggestion would be to install the latest version which is due out next week on 1st February 2010, if you are able too.
Laters!