You need to consider:
- how many user will run jobs on your galaxy server
- how often will they run jobs
- what kind of jobs
- what kind of data and source of data, e.g.: bacterial genomes / mammalian genomes
and keep in mind: The Galaxy server itself doesn't need many resources. The tools you execute require the resources. Hence no matter whether you execute the tools on the command line or via Galaxy, you need the same resources.
Also, jave you checked the "Galaxy Choices" page: https://galaxyproject.org/choices/