upgrading a server is always a good thing but this must meet your capacity/growing plans. If your actual server’s RAM is enough for managing the email traffic and what is lacking is only the hard disk space, why are you also considering the RAM upgrade?
If you are getting the additional 3Gb RAM for few bucks go for it because this will make your server more “open” to future growths but if you are paying that 3GBs upgrade a bunch of money don’t go for it.
You may also consider what is the warranty/service status for your server, if it has still long life to go you can upgrade it, if instead your warranty/service is going to end in few months and the money needed for updating your warranty/service is too much, you may consider buying of a new server (compatible with the hard drives you are going to buy) that will come with at least 2 years warranty.
Another solution may be buying an affordable NAS that you can connect to any server, the actual one or the future one.
Once again, everything depends on which plans the company has; rare hardware buyings+estimated growth=upgrade the server; frequent hardware buyings=don’t upgrade but wait; you need capacity soon but ignore plannings=buy NAS.
The above are only example of considerations you have to keep in mind.
I hope this helps.
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