All three of the above raise very good points. I had never heard of memcached and it sounds excellent - will look into it futher.

The other two comments brought me back to the real world: In any site, chances are that, even if you have many people creating new groups (as would be the case with my current project), it is still tiny in comparison with the number of people doing reads.

Hence the need for instant cache updates is reduced.

Many thanks for the suggestions - I will sleep easier tonight.