Cotton - I'm always amazed that on a tech-savy board there is so much discussion of client-side SPAM filtering and virtually no discussion of server-side filtering.
My current and previous ISPs both use the server-side filter "Postini," and I get at most 1-2 SPAMs a day, plus the satisfaction of knowing that I have made the entire email system more efficient by stopping SPAM one step closer to the source.
If I were to need a new ISP I would put server-side filtering on my list of absolute requrements, right along with reliability and security.