Using photoshop, i put together this banner:
Photoshop is amazing for creating depth and a sense of perspective even in simple stuff like this, but if you're going to make an animated banner, I have to disagree with ebrown and say that Flash is a good way to go... but if you do so, make sure you export the Flash file as a .gif, else you'll have trouble finding a website that will allow a .swf banner.
Even if it's flashy, which it should be, overcomplicate it or jam it with too much info and it'll put off your viewer. Also, if it's animated, don't let it move too quickly, since except for the most savvy computer-users, they won't be able to keep up with the animation. Try to make it aesthetically pleasant while providing them with just enough information to attract your clientele. It's hard to juggle, and I daresay I haven't nearly gotten the hang of it, but it's certainly fun to try!
Best of luck, and if you need help with flash don't hesitate to ask.