Backup 5.x and update to 6.x or 7.x without site interuption

Hello,

I work for a small environmental non-profit downtown SF that needs a Drupal Expert!

We need to perform a Drupal upgrade from version 5.x (our current very out of date version) to 6.x (or possibly 7, the latest version about to come out). It is my understanding that Drupal only supports two versions at a time, so when 7 comes out, it will only support 6 and 7, leaving our version open to security issues or not working at all. I am not a programmer or developer, yet maintain the website and do basic content updates. We've been getting more and more errors lately, my WYSISYG editor is freaking out and I'm losing all prior established rich-text formatting in current pages while I update them. I foresee these problems only getting worse the longer I prolong this upgrade.

Our website is currently live and being hosted by Godaddy.com so we want to make sure we minimize the downtime of the site. It is extremely important that the site be backed up in the event that the upgrade(s) fail and you must have experience with that. I'm happy to email more details and talk about our current Drupal issues if you’d like.

If you have experience with this type of project, we're looking for you! Have you ever done an upgrade with Drupal from two versions back? What would be your time and cost estimate be of a project like this for us? (Our website is about 150 pages in length and is www.alcnet.org)

Thank you very much in advance for your time!