I've tested with another normal map (http://www.sierracosworth.nl/gamedev/_normalmap2.png) and the artifacts are practically gone.

Your first normal map is extremely noisy. Looks like it's been converted to a palettized gif and back or something. Not to mention those strange lines are also there (very obviously if you load it into an image editor and up the contrast)..

Thanks, I wont focus on the artifacts then. Better spend my time on finding out how to make good normal maps based on existing textures. Any suggestions/ pointers?

Clear regarding the hemisphere ambient, I'll have to dig into the "lerp" theory, the result without *0.5 + 0.5 actually isn't what I expect/ look for, compared to without it