Thanks for the suggestions. I have a few comments about some of these:

We run a site based on Elgg 1.5

Upgrade to at least 1.7 as soon as possible. Elgg 1.5 is extremely outdated, contains many bugs, and contains many security issues!

1). use Memcached. (but there's a bug if you use it, and we still haven't fixed it.) 

Memcache support is experimental at best and there are significant bugs in it, especially in 1.5. It's not production ready. We've disabled it on the community site and I highly suggest *NOT* using it on a production site.

2). convert most of Mysql tables to Innodb engine, include entities, metadata, metastrings, objects_entity, etc. (very important if your site has large data. but be careful with those tables having full text index.)

This is a dangerous game with only a marginal payoff. Elgg's search uses full text indexes on many of the tables. The only table you can safely change to InnoDB is metastrings and metadata. I'd be interested to see the stats for performance gain by switching to InnoDB.

3). install an image cache server, like Squid.

This can definitely help for static images.

4). customize Elgg if you can.

Depending on what the customizations are, this may or may not help with performance. When you do customize Elgg, be sure to do it correctly with plugins and not by changing core or you'll have a difficult time upgrading.

For reference, here is our MySQL stats:

Uptime: 13 days 44 min 2 sec

Threads: 7  Questions: 345152925  Slow queries: 5655  Opens: 200549  Flush tables: 1  Open tables: 64  Queries per second avg: 306.573

As I said in this post, I'll be posting to blog.elgg.org with details on how we greatly improved the speed of this site with very few changes to the server.