Welcome to WebmasterWorld, jkadmin! That's a fair number of posts, but not a pageview number that should overwhelm your beefy server. One key question - in the vB admin panel, how many users do you see on during your peak periods? And does the page load time get a lot longer during periods of high usage?
The 15K drives are good, and the fact that your server utilization is staying below 1 is also good. I'd check memory use, too, but the 8GB sounds like enough for your load.
I'm in the process of rolling out a speeded-up vB forum, and the lead developer working on it found a lot of little things that cause slower loading pages. I suggest using a tool like Fiddler or similar to check every element on the page for time to load. ySlow is another useful tool. You may find ad code, analytics code, images, or other content that is slowing you down.
One time-honored approach is with vB forums is to try to minimize the queries that are needed to build a page. There are plugins that will display the number of queries and page gen details. If you are displaying info like "who's online" that aren't necessary for your community, turn those features off.
I'd also check vB's own forum for discussion of this topic. And, if all else fails, throw hardware at the problem. Splitting your web server from your database server is often the first step, followed by load-balancing more than one web server. In your case, though, I think you should be OK with your hardware unless you are getting short bursts of volume that overwhelm it.