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There have been lots of threads on the forums dealing with these questions, so I'll start by giving you a reading list. ;-)
These are just ones I can remember, which probably means I contributed to them - but I'm sure there are lots of others. There is a wealth of great content buried away in the archives which I don't thing gets picked up when you just go into an active forum and filter - so it's worth going to the archived forum and scrolling through the topics in the Maps and Geocoding forum, too.
Arbitrary rectangular bins:
Approximate square bins:
Circles and Rings:
*** This one started out about something completely different and got onto radius selection about half way through.
Robert Mundigl has also done some great posts on his Clearly and Simply blog site about this topic, and I did a follow-up series shortly after Robert's article about "site catchment analysis" with Tableau version 6.
Finding points within DMA boundaries, etc really needs a spatial database. Several of the links I've pointed you at talk a bit about what you can (and can't) do with Tableau with spatial databases.