RAM/Memory - when to upgrade VPS?
At what RAM/Memory usage is it time to upgrade a VPS to keep it running at optimal speed? I am trying to maximize the accounts on the servers without slowing the users down.
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Without SWAP in Munin there is free memory and then there is memory which is being used as cache. At some point the cache will get smaller because apps use more RAM. In your experience, what's the % of RAM that a system should have available for free/cache before it becomes noticeable slower?
I also have one VPS (SSD) which has 2GB SWAP on top of 8GB of RAM. The swappiness is set to 1, but with the 6GB it used to have it started swapping like crazy while it still showed 25% of free RAM...so on that one I'll have to leave extra room. Any guesses if it'll still swap at less than 75% usage or if it'll swap at a different point since the ram/swap ratio is different?

Thanks
Cached memory is used for the filesystem, so it entirely depends on the applications.
Why don't you monitor performance of the applications themselves and upgrade when needed?
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@ServerHunter
You are correct. Although these are client sites and I am not in the habit of browsing the sites constantly to see any changes in performance. I'd rather have a guideline where I should start monitoring a server more closely.
50% free is more than fine, but 5% is probably trouble. Or is it more in the range of 2GB of cache is sufficient no matter how many sites or what the RAM? 
In hindsight I didn't ask the right question. The question would probably be have been better as: when do I stop putting accounts on a server once they hit the maximum of the VPS configurations available. 
I ran into an issue where there was basically upgradeability but I had a good pricing for the current plan and that wouldn't have carried forward according to first level sales reps. I was able to find a solution for that VPS with second level after all, but don't want to go there again.
Also on some maxed out VPS plan I want to make sure I have enough room for the sites to run well so I don't have to move accounts off.
Most sites are WordPress based.

The IT community developed an 80% rule several years ago. Anytime a resource averages 80% of the total availability, its time to take a serious look at upgrading. Because RAM and CPU spike and an extended spike can cause slower responses by the server, I like to use more of a 60% average to start evaluation of the hardware and begin a more in-depth monitoring of the server. This can tell you in a fairly short time, if instead of an 80% average, you need to be at 70% average for example.
Tony Holloway
Delivering Custom Hosting Solutions Since January 2004
madRooster.com
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Thanks, the 80% sounds like what I had in my mind. 
The starting to look at 60% makes sense. That way I my have avoided the issue I had with one server that started swapping crazily at 75% even with swappiness at 0.

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