It completely depends on how CPU intensive your particular game is, how efficient whatever engine you're using is, and likely how many users you have online.
We've had speed issues on top-end Xeon servers that are only running a single MUD, but that doesn't happen without an extra large number of players on (500-600+) during an event when they're all running around quickly doing stuff. Our Rapture engine is also running interpreted code, which is, of course, inherently slower than native, though nicer to work with.
--matt
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