I say go for it. The experience you gain will be great!
Years ago, wedding photographers only had the option of film. People today would never consider covering a wedding with "just" a film camera. But in those days they made do and they turned out great photos. I have proof with my own wedding photographs!
I've read that today's megapixel equivalent in professional film cameras is 6 megapixels. The Canon 450D has far than 6 mp's! Moreover, the 17-55mm lens is more than capable.
To build up your confidence prior to the day of the shoot I would suggest the following:
1) Practice. A simple way of doing this is to tell your family that you'll be shooting them all day in their natural setting. Tell them to act natural and to pretend that you're not there. After the family practice shoot, review your photos and see where you can improve.
2) Read the thousands of wedding procedures and tips on the internet. Pay particular attention to the photos that are taken and try to incorporate them into your 'arsenal'. Here is a short PDF to get you started:
Guide to Wedding Photography.
3) Go to your local book store and buy a book on the do's and don'ts of wedding photography. Here's one from the great Dummies Series:
Dummies Guide to Photographing Weddings Using Your Digital SLR.
4) Explore the multitudes of YouTube videos on wedding photography.
5) Go to the web sites of actual wedding photographers and take note of the types of poses they have their clients do.
Interestingly, as I compiled this list I realized that years ago, aspiring film photographers only had options '1' and '3' to help them get up to speed. Now we have so many options through the power of the internet.
Don't lose this great opportunity! I say go for it! Who knows... you may be asked to do a paid assignment before you know it!
Good luck!