I want to create a large database of GPS coordinates that can be queried by saying \"Return all coordinates that are within \'n\' metres of [this coordinate]\".
Many database systems have function for working with geospatial data.
Here is comparsion geospatial functions between SQL Server 2008, PosGIS and MySQL
http://www.bostongis.com/PrinterFriendly.aspx?content_name=sqlserver2008_postgis_mysql_compare