The content, whether humorous or serious, should never include material that may be offensive or embarrassing. This is a toast, not a roast, and not the time for off-color or extremely personal remarks. Keeping in mind that the objective of the toast is to honor the bride and groom, will be a way by which to judge appropriateness. Preparation should involve not only selection of materials, but review and rehearsal. Offering the toast is an important honor to the giver and the receivers, so it needs some time for review.
Note cards are perfectly acceptable as a way to remind the presenter of the material.
It is, of course, preferable, if possible, for the material to be memorized. Memorization only works if it doesn't make the material mechanical sounding. From a technical standpoint, it is advisable for the toast presenter to check out the electronic equipment. These too are the professionals who should be asked to quiet the guests before the toast is actually offered. Of course, the presenter will, need to remember not only to bring notes, if there are any, but also a glass of champagne or wine with which to toast the bride and groom.