I am currently running a plain ol html/css site created with dreamweaver. I am not experiencing any problems with the site but my users are asking for a the ability to update various pages on their own which is why I am looking at CMS.
OK, but make sure your users are reasonably computer-savvy. Updating pages, even with a CMS, is not the simplest thing. A CMS gives you lots of flexibility in what to do with a page, i.e., plenty of rope to hang yourself. Are these "users" the site administrator(s)? You will probably have to give out the master password to the site to anyone allowed to edit it, so you don't want to spread that too far.
A Wiki is designed from the ground up to be updated by many people (general public or
password-controlled), but I'm not sure you will be able to do the specific layouts you want. Have you considered just what parts of the site need to be updated by your users? If it's all text, and relatively small segments, you might have a password-access form to update certain pieces of text, storing it in a database or even a flat file. That would be done in PHP, but would give users fewer opportunities to shoot themselves.
The site is for a nationally known marching band and we get perhaps 700k hits per month. Even so, I imagine that is pretty small volume. I am thinking that the traffic would be the main concern for cpu overload? The site is pretty large content wise. Would that have an impact on cpu?
A CMS (or even a hand-coded PHP site) will definitely use more resources than a static HTML site. The only way you'll know for sure is to try it out. If LP suspends your account due to excessive resource usage (CPU load), you'll know you went too far* and you'll have to fall back to your old static site. That would be a shame to have put all that work into a CMS, only to be unable to use it without upgrading to a VPS or even dedicated server! At least, you'll have learned how to work with a CMS.
*I wish, I wish, I wish that LP would give its customers a way to monitor their resource usage, so they could take corrective action before being shut down.
I am leaning towards Drupal. Do these packages typically continue running properly over time as web host components evolve? For example is there much of a risk that the site will break the next time php is updated?
Drupal is considered the Mercedes Benz of CMSs. They do require that you periodically upgrade the Drupal software as PHP and other components are upgraded. As almost everything is kept in the database, this is generally not too painful. I presume that Joomla and Wordpress behave in a similar way. Any major package like this will keep up with PHP and MySQL changes. Small, poorly supported software might not get upgraded very quickly, but I wouldn't worry about the major players.
By all means, make a test installation of Drupal or another CMS, and play with it. You may find it wasn't as much work as you feared to bring up your site using the CMS, or you may find it overwhelming (especially Drupal).