MySQL high CPU usage (MyISAM table indexes)
SUGGESTION

Your key_buffer_size is only 16MB, but your MyISAM indexes total 59MB. In a highly-trafficked website, I can see index pages getting kicked out of and pushed into the MyISAM keycache frequently. This could have an affect on indexes being updated and overall write performance on the MyISAM tables.

You could just bump up key_buffer_size to 64 MB.

You must add this to /etc/my.cnf

[mysqld]
key_buffer_size=64M


You don't even need to restart mysql. Just run this:

SET @MB = 1024 * 1024;
SET GLOBAL key_buffer_size = @MB * 64;


You can also convert the row format of all your MyISAM tables. This will just about double the disk space used by will increase read performance 20-25%. I wrote about this before as well:


Which is faster, InnoDB or MyISAM?
Best of MyISAM and InnoDB


Here is a script to convert all your MyISAM tables to Fixed Row Format

SQLSTMT="SELECT CONCAT('ALTER TABLE ',table_schema,'.',table_name,' ROW_FORMAT=Fixed;')"
SQLSTMT="${SQLSTMT} FROM information_schema.tables WHERE engine='MyISAM' AND"
SQLSTMT="${SQLSTMT} table_schema NOT IN ('information_schema','mysql','performance_schema')"
mysql -uroot -p -A --skip-column-names -e"${SQLSTMT}" &gt; /root/MakeFixedRows.sql
mysql -uroot -p &lt; /root/MakeFixedRows.sql


ALTERNATE SUGGESTION

Given you have a total of 488 MB of of MyISAM data and indexes, I would like to suggest the following: Switch Everything to InnoDB. I have two reasons: 

REASON # 1

MyISAM only caches indexes. InnoDB caches data and indexes. See my post : What are the main differences between InnoDB and MyISAM?

This will reduce disk I/O for reading data and indexes. In your particular case, you can easily handle an InnoDB Buffer Pool of 512M and basically fit everything into RAM.

REASON # 2

The MyISAM storage engine cannot access multiple CPUs. InnoDB can. In fact, I have many posts on tweeking InnoDB for this:


Jul 23, 2012 : How to get the most out of MySQL on a QuadCore machine with 16 GB of RAM?
Sep 20, 2011 : Multi cores and MySQL Performance
Sep 12, 2011 : Possible to make MySQL use more than one core?
May 26, 2011 : About single threaded versus multithreaded databases performance


EPILOGUE

I would like you really consider the conversion from MyISAM to InnoDB


When to switch from MyISAM to InnoDB?
Any gotchas at all with converting from MyISAM to InnoDB?
InnoDB vs MyISAM with many indexes