Storing and accessing GeoData( Latitude and Longitude ) in PostgreSQL 8.3

Hello,

I have a lot of GeoData( Latitude and Longitude ) in my PostgreSQL Database. Is store them as numeric at this time.
But now I want to access the data and want to have all Datasets around a Geographic point. eg: "within a radius of 5 km"...

Is there a simple way to do something like this?

Thanks for your Help,
Stefan Sturm