1. linux is a relatively light-wt OS so it doesnt require lots of memory. but it is for web service so its work load may vary greatly due to demands. its better to have more memory. add 2GB for a total of 4GB
2. windows is a heavy OS n requires loads of memory. to run a windows server, it needs at least 8GB n may be up to 16GB to be on the safe side
3. storage server does not require much memory either. if u want to lower utilization from 85% to 60%, add 2GB for a total of 4GB