NOAA have some DEMs. (Digital Elevation Models). Notably ETOPO1, topography and bathymetry with a resolution of about 1.5 km, available as a nice shaded relief map as well as a DEM. http://www.ngdc.noaa.gov/mgg/bathymetry/relief.html

This website http://desktopgisbook.com/data has shapefiles for national borders (not up-to-date with South Sudan mind), and locations of 606 major cities worldwide.