What version of iWeb is it? In the first version or so of iWeb this was an extremely huge problem for me, it's one of the reasons I stopped using iWeb.
If the site is fairly small, the best solution is to create it from scratch. If you can upgrade to the latest version of iWeb, I've also heard that '08 and '09 are both far better on the front of having much more reasonable file names.
Another potential solution, which I came to a few years ago, is that rapidweaver http://www.realmacsoftware.com/rapidweaver/ makes much more reasonable filenames.
Another potential solution, if you have access to it, is using Dreamweaver. Dreamweaver templates rock a bit, and while it's more involved than iWeb, it allows a lot more control over file naming.
If the site is fairly small, the best solution is to create it from scratch. If you can upgrade to the latest version of iWeb, I've also heard that '08 and '09 are both far better on the front of having much more reasonable file names.
Another potential solution, which I came to a few years ago, is that rapidweaver http://www.realmacsoftware.com/rapidweaver/ makes much more reasonable filenames.
Another potential solution, if you have access to it, is using Dreamweaver. Dreamweaver templates rock a bit, and while it's more involved than iWeb, it allows a lot more control over file naming.