The cache hit percentage is the percent of requests that call the accelerator (ultimatebb.php)
53% is great cache percentage
There isn't a magic light that will blink about an upgrade. I think when you are getting over half a million page views a month, your host might get grumpy. They might start trying to coax you to a PHP/MySQL based board at that point. That of course depends on the number of members, topics, server, etc.
