I'm really really really new to nginx Actually right now my site's on Apache because I was using it to debug Google PageSpeed woes for everyone.
Your CPU and memory are both totally awesome low. On my rather larger site, I have to have my memory set to 800MB just to have a prayer of running (it's on 1G right now thank you reddit). Memory usage on WP is a funny thing. If you're using caching, your memory will actually go up, but your CPU will go down. Since our default nginx comes with PHP 5.2 (which is also not the best for memory), I'd actually stay on apache right now with PHP 5.3 and home-brew xcache.
Then I'd set W3TC as follows:
Page Cache: Disk Enhanced
Minify: Enable, Disk, Manual
Browser Cache: Enable
Object Cache: Enable, xcache (or apc, something other than disk)
Page Cache, I usually take the default settings. I have a couple pages I force not to cache, like /wp-content/uploads/, so I can block hotlinkers.
Minify: HTML check everything. JS and CSS I only check 'enable' (since I have weird results when I compress and join them, right now it's off).
Object and Browser Caches are normal/defaults.
I very rarely even consider DB caching, since my sites get enough DB traffic (comments etc) to make it totally worthless. ONE site is, essentially, read only, and that I use it on