Vander, I'm going to have recommend you don't follow Phil's advice in this instance.
I've done some speach writing and composition in the past and still do on some occasions, so here's how I'd apporach it.
It's a large gathering, prestigous guests etc...that is telling you 2 things: Stay away from personal stories that only a few know, as not many will "get it" and I would not go the humor or lame joke route to get a laugh.
Keep it brief, sincere and from the heart.
Most of these people are not intimate friends, so they are not going to be interested in a 10-15 minute long winded best wishes speach.
If you're not introduced, introduce yourself, describe the length and quality of your friendship with the groom and how much him and his happiness mean to you, speak to him directly, even though you are addressing the crowd.
Describe what a special person the bride is and how happy she has made your friend and touch upon how happy they are as a couple and how well they compliment each other (you may have to make some of this up
), again, speak to her and them directly even though you are addressing the crowd.
Wish them the best, offer up a toast to their happiness and longevity and let the party roll from there.
ETA <5 minutes, everyone's happy and drinking, eating and carrying on.
If you have a certain story you feel you should share, by all means do, just have the structure of the story down pat, so you can deliver it clearly and concisely, so those that don't "know" about it, will still find it amusing.
Good luck, just be yourself !