Using your example couldn't you do something like this

SELECT h.*

FROM zipcodes g

JOIN zipcodes h ON g.zipcode <> h.zipcode

AND g.zipcode = '10028'

AND h.zipcode <> '10028'

WHERE g.GeogCol1.STBuffer(20 * 1609.344).STIntersects(h.GeogCol1)

--STBuffer would...