Since you're already getting help with the server setup let's delve into what's being run to make you have issues so much earlier than the "average" site.
Do you have any hacks installed? Multiple templates? Custom code you wrote?
When you say 600 memebrs, is that including guests or not? What's your total activity and guests to member ratio?
Have you run any sort of slow-query report to see what (if anything) is getting caught up?
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