Memo fields (text fields of unknown size), are not indexable and therefore are invisle to DB based searches etc.
Gary, your full post, from which I took the above quote, is a bit over 400 characters in length. Yet, if you use the Search function in the forums to search for the phrase "text fields of unknown size," I think you'll discover that your post is NOT invisible to DB searches. That's because "not indexable" isn't quite the same thing as "not searchable."
Of course, one might also note that the static pages you are recommending aren't indexable either?
The choices made in RDB design are invariably between speed/efficiency and flexibility/maintenance. Thirty years ago we had no choice but to program for speed, because the hardware we could throw at a problem was sorely limited. Today, however, Iron is cheap and getting cheaper. If your Product Descriptions need more than 255 chars, there is very rarely any reason not to do so.
what if the dynamic page is only accessbile through a web form and offers good advice to users?
surely then its acceptable to create a static version of it and link to it through a sitemap?
Static pages are one alternative, but they may not be the only alternative (and they are rarely the best alternative).
The problem with a static reflection of your database is that it breaks the most important rule of relational DB design -- never duplicate data. If you store a product price in two different places, it's twice as much work to update the price and sooner or later the two fields will get out of sync. A static HTML page is just another way to break this rule, because any changes in the underlying data will require you to rebuild at least a portion of the static pages (often ALL the pages). Open the door for human error, and it WILL move into your home.
Instead of creating a static page to replace the information available from a web form, you might want to investigate ways to make the underlying script perform double duty. For example, change the script so that it accepts parameters from either GET or POST, depending on how it's called, and many small forms can then be replaced with links that include a query string. It sometimes takes a bit of creative programming, but even larger forms can usually be converted to dynamic links, eliminating the need for static pages.