google maps - Slight translation with coordinate-transformation from Gauss-Kruger to WGS 84. Possible Reasons?

I have a number of points in Gauss-Kruger, Zone 3 that I want to convert to display them within Google-Earth. Assuming WGS84 for Google-Earth the transformed coordinated differ from the original ones about 140m to the north-east.

I am doing the coordinate transformation using the "PROJ.4 - Cartographic Projections Library" (http://trac.osgeo.org/proj/) with the following proj4 strings:

Gauß Krüger, Zone 3:

+proj=tmerc +lat_0=0 +lon_0=9 +k=1.000000 +x_0=3500000 +y_0=0 +ellps=bessel +datum=potsdam +units=m +no_defs 

WGS84:

 +proj=longlat +ellps=WGS84 +datum=WGS84 +no_defs - "

Could somebody check if my proj4 strings are correct (especially WGS84 for Google Earth) or suggest other causes of the error?