Bridge Over Troubled Water

I've made a map combining texture and normal mapping, applied upon a very low poly model (my first idea required more than seven millions of polygons!). I'm still not satisfied because the squares that make up the final map can't be perfectly aligned, and the space between one and another can be seen. I've tried with one unique mesh, with rather big textures (8192 x 4096) but it seems it's too big a dimension for the engine (i.e. I can't see anything on the screen...)
I'll make some other tests, but I hope this part will not take too much time.