Ummm... being a skydiver I'm not inclined to be afraid of many things. I just don't spend that much time on here.
Dropzone.com lives on a cluster of 5 machines hosted by Gossamer-Host. Alex, will do a much better job than me of telling you the server configuration. Feel free to contact them.
All in one mySQL database and I have no issues with slow-downs. The servers have quite a bit of redundancy built in.