Dude, frack no. Having everything in a database means you are tied to the speed of disk, instead of the speed of ram. The biggest issue with being purely ram resident is handling multi-threaded updates across your ram resident dataset.
No. You journal the changes to disk. You therefore only need disks fast enough to keep up with the journal. If you get into a situation where your disks can't keep up with the journal, your site is probably big and popular enough that you can afford to hire DBAs to go from there.
For a site this small, there's absolutely no reason the entire database shouldn't be cached in RAM. 32GB of RAM costs about $800. That buys you plenty of time to not have to worry about caching, and instead gives you more time to work on interesting features. For a single-person operation (or even a few people), you have to spend your time wisely.
But of course, any sane database will cache as much in RAM as possible anyway. But instead of dying when you run out of memory, it just evicts the least recently used page.
You could always use Erlang...