To be honest, I believe system administration is an all or nothing game. I would be asking myself questions similar to:
- Is this server of critical importance to the success (of the business/organization/service your trying to provide) ?
- Can you commit significant resources (time, money, effort) to managing this machine?
If you can answer yes to both, then I would probably suggesting reading something similar to:
The Practice of System and Network Administration, Second Edition (ISBN: 0321492668)
This book is an awesome read on "big picture" concepts whilst typically remaining operating system agnostic. Once you are all over this book you can look into OS specific resources. I say this primarily as most people in the CompSci game can read manuals and work out how commands work. Getting the principals right in mind before making changes is of uptmost importance for long-term system maintainability, reliability and longevity.
Alternatively, If you answer yes to 1 and no to 2 then you're probably going to be better off getting someone specific for this purpose or even outsourcing the on-going management of the server an company which specializes in this area.
Another option which may be more cost effective (depending on what services you're running on the machine) would be to simply move the hosting accounts to virtual hosting environment where the on-going server management is taking care of by a third party. This obviously has the draw-backs that you're going to co-exist in a shared environment with other customers on the same machine which will naturally diminish your control over the environment. This may or may not be a viable solution.