Best Mac ISP??

Since the Mac uses international standards to connect to an ISP and pretty much every ISP also uses those standards you will be hard pressed to find an ISP that won't work with your iBook.

Some ISPs can't provide any support (i.e. answer questions) for the Mac OS, but the good news is that you rarely need that.

It won't make any difference, except for maybe whether the ISP "supports" Macs or not - meaning they may or may not be obliged to help you with any technical issues on the Mac should they arise - does not necessarily mean that Macs won't work with their service. If you're looking at broadband, you're likely limited to DSL through your local phone company, or whatever cable company monopolizes your community.