I would indeed switch to InnoDB. Table locking/row locking has long been discussed by many. I would always choose InnoDB hands down. However, there is another profound reason for choosing InnoDB…CACHING.
While most people boast that MyISAM is faster for reads, most people forget that the many cache for MyISAM, which is called the key cache (set by key_buffer_size), only caches index pages from .MYI files. It never caches data pages. It has an official maximum of 4GB in 32-bit Systems. 8GB is best maximum for 64-bit.
The InnoDB Buffer Pool caches the data and index pages. Depending on the server your have, you can cache up to the entire dataset in RAM. You can tune InnoDB for up to 80% RAM and 10% for DB Conenctions, and leave 10% for the OS. This is true even for different operating systems.
I have recommended these things for Drupal customers with marvelous success. It applies to WordPress just as well. I have provided DB support for clients with WordPress. Same improvements.
You can always configure memory for InnoDB more effectively that you can more MyISAM. There is always a way to tweek InnoDB to suit your performance needs. As your data grows, it will eventually become a requirement.