1. You should be able to upgrade iWeb 06 to 08 from the Downloads section of the Apple website. However, at the present time and because iLife 09 has only just been released, if you want to upgrade to this, then you would need to order it via the Apple website. You would have to pay for this.
You could start by upgrading from 06 to 08 for a start and then consider 09 when all the bugs have been fixed in it.
2. To be able to use iWeb, no, you do not need to have a MobileMe account. If you wish to publish, you need to select the Publish to a Folder option from within iWeb and this will publish your site to the Sites folder and then you can upload this to your chosen server using Cyberduck which is free ftp software program.
You can sign up for a MobileMe account and it would cost you $100 per year for which you get hosting space and you can also link your domain name to your site too. If you decide not to purchase a domain name however, you MME url will be
http://web.me.com/username.