Example of two smaller server types...
1. 4 year old quad-core 2.6GHZ Xeon - slow hard drive, 8 GIGs Rams - lots of add-ons and even other small sites and CMS on the same server...1.5 million posts.
Serves up to 3,000,000 pages per month with no sweat. Never saw load above 2 for any period of time.
Usual peaks are about 1,000 online at once (according to XF stats).
2. Tiny VPS - 1G Ram and 1CPU or less. About a dozen add-ons and one instance of WP
Serves 200,000 pages per month and load is very low even in the single slice of CPU.
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So basically you want to figure out what you have in terms of traffic, what you expect to happen and then you can make a informed decision. Only a tiny subset of forums would ever need the kind of iron Slavik is talking about.