1.5GB of RAM is not that much of a resource pool for ~50 different websites. We have sites hosted on 4GB of RAM VPS.
It all comes down to the types of sites, databases, traffic/simultaneous visitors, optimization techniques and the underlying hardware of the hypervisor nodes. In general, you could get a VPS that's much more powerful than a dedicated server for a fraction of the costs, if you know how to pick your underlying hardware.
The great advantages of a VPS are the scalability of the service over a full fledged Dedicated Server. As your company grows and your customers grow your needs will increase and being on a VPS should allow you the ability to scale quickly when your usages demand resources. With a VPS you do run the ability to have a greater profit margin, being a small company, as you grow moving into dedicated hardware can help as you will have the say so on what hardware/services are implemented in your enviroment which you can then fine tune to your specific customer needs.
Since your using CentOS I am assuming you are most likely using cPanel, if this is the case when you plan on moving to a Dedicated Server you might be interested in utilizing CloudLinux with CageFS. This can help manage your customers usages on your server so that you can utilize your hardware to it's fullest.
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The answer's already been stated and I'm not going to reiterate what most people have correctly stated.
However, you don't necessarily need to upgrade to a hybrid or a dedicated server. Your current specs are more towards the low-ish end for a VPS and given the industry standards now. When you do outgrow your current specs, you can always upgrade to a higher spec VPS without a substantial increase in cost, as opposed to a dedicated server.
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The server specs you have right now are pretty decent but also you won't need to upgrade to a dedicated server for a while as long as your current VPS can without what your site is doing now. Even in the future you may not need to upgrade to a dedi but rather you maybe able to just upgrade the current VPS you have now x
I think what you have right now sounds good for the sites you have but like everyone else has also said you have to keep an eye on the resources of the server. You need to make sure you keep up with the server.
It's exactly depends on the resources that they are use, for example if you have 50 pure HTML based websites with no databases then you don't need it. it depends on your usage!
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