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.
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.