Ok, Pyree. I think you are right. My internet conection sucks and definitely is the bottleneck. I tested several times and I can not get over 2 mbps. The wireless is sometimes close to 140 mbps, some times 40 mbps, and sometimes even 12 mbps, but allways over the speed of my internet conection, so I can not blame wireless N for this.
Now, regarding to the range, I just moved a little the router from its original position (not so far anyway), and I'm now obtaining more consistent results. Not a big difference between them but allways a better signal from N than G.
So I really appreciate the time you took for answer my question and I´m going to close it.