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Not sure which one is the best, but we've been using liquidweb dedicated for about a year (or so) and couldn't be happier with them. I would also recommend checking out www.webhostingtalk.com - there's everything about hosting that you need to know.
its not just about the server. from your specs you should consider the following: - have you installed caching application to help with the server load - optimize your mysql for performance - optimize apache config - considered increasing memory? - check if any vbulletin add-on has caused your load to spike - analyzed your average load? that's some things you can look into, i run a big community as well and there are lots of things i went through before knowing this stuff...