@Kate, to pull this back out of the techie-vs-techie stuff, were any of the above posts helpful?
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As a 2nd-pass at helping:
Disk I/O tends to be the first thing that limits WordPress site speed.
Your top results "load average: 0.40, 0.37, 0.31" and "Mem... 35160388k free" show the server CPU and RAM aren't fully-used. To me that also indicates your disk I/O is the issue.
The simplest caching setup to reduce disk usage is probably the Memcached object cache, http://wordpress.org/plugins/memcached/.
Two of the other plugins mentioned (Tribe & W3 Total Cache) have that as part of their functionality. Any of the 3 should buffer your speed problem quickly, then you can proceed with more detailed analysis.