There was a great write-up of a talk on SSL/TLS performance at Google linked here a few months back (http://unblog.pidster.com/imperialviolet-overclocking-ssl, HN discussion at http://news.ycombinator.com/item?id=1485425)

Quoting from that, "On our production frontend machines, SSL/TLS accounts for less than 1% of the CPU load, less than 10KB of memory per connection and less than 2% of network overhead."