A timely post. We've been doing a lot of work on this recently so performance is top of mind. Here are some stats.
On a server with the following specks: Dual 2.6 GHz dual Core CPU, 2 GB RAM, 2 x 146 GB
SCSI HD RAID 1. We are getting the following results;
2.2.2 handles around 50 page view per second on that server.
2.3 (not yet released) handles around 200 page views per second on the same server.
The performance code for 2.3 will likely make it's way into the trunk branch in the next few weeks if you want to try it out.
So as of 2.3, a server like the one above will be able to support around 1.7 million page views per day. If you need additional performance then you need to run multiple servers with load balances in fount of them.
Re running separate database servers, this is pretty trivial to do - just point the database connection to a new server, however unless you are running a heavily dynamic site there isn't much value in this as all the pages (as of 2.3) are statically cached, hence there are no calls to the database for most page loads.
As for some stories, we're working on a few very high load sites, one is going to have a two servers with a load balancer + a dedicated database server, and another going to have 10 - 15 servers controlled by load balances. We'll write up a bit of a post mortem in a few months.