Postal code radius searches

I've been struggling with this problem for a while now and I can't seem to
find a solution. I have a postal-code database, currently populated with over
76,000 United States ZIP codes. Each record contains, among other things, the
latitude and longitude for the postal code. I have a stored procedure that
calculates the distance between any two points on the globe. I'm trying to
figure out a fast way to locate all of the postal codes within an arbitrary
radius of another postal code.

The brute force method requires a sequential scan of all 76,000 records
looking for those that fall within the specified area. A more
high-performance method would be to pre-calculate the distances between all
postal codes (possibly limiting the distance to save space). However, this
requires more than 76,000 ^ 2 database operations. On a 1 GHz box, I
calculated this would take nearly one year complete. It would take twice as
long if I wanted to create a second cache for city/state searches.

Does anybody have and tips on solving this issue? Is there any sort of
complex index I could create based on the results of an arbitrary stored
procedure call? Maybe some custom C code?

--
Milo Hyson
CyberLife Labs, LLC