Creating circle in PostGIS?

I'm using PostGIS 1.5.2, with geometries in SRID:900913. I need create a circles using a list of points as centre, with a radius of 600 kilometers. I'm using this query:

INSERT INTO circles (geom) (
   SELECT ST_Buffer(point, 600000, 'quad_segs=8') 
   FROM points
);

But circles created have not 600 kilometers of radius (radius is near this length, but not exactly).

Are there other methods to create circles in PostGIS?

NOTE: Information represented is from Spain. Correct projection is 4326, but client use Google rasters, so I am storing data in 900913 to avoid reprojections and increase performance.

Answer

Try this:

 SELECT ST_Transform(geometry( 
            ST_Buffer(geography( 
                ST_Transform( point, 4326 )), 
                600000)), 
            900913) FROM points`

This flips into geography then uses the inbuilt SRID selection to (ironically) flip back into geometry where a good planar buffer is run then flips back. The trouble with your approach in Mercator is that Mercator doesn't preserve distance. Using a more appropriate local projection gives better results, and that is what happens in the above method.