Over the last week or so I've been helping the Product Support folks at Microsoft write a new KB article which discusses some of the things you should take into consideration when setting up database mirroring.

It discusses memory, CPU, I/O bandwidth, network bandwidth, and transaction volume. One of the most interesting things it discusses is exactly how many worker threads are required on the principal and mirror for each mirrored database.

Check it out at: http://support.microsoft.com/kb/2001270 (the formatting looks a little weird in the final online version, assuming they'll fix that up).

Enjoy!