Alfredo,
See the noise thread:
http://www.birdphotographers.net/for...867#post759867
Basically, smaller pixels act pretty much the same as a TC. So if you get a camera with pixels 1/1.4x smaller, it is like you added a 1.4x TC to your existing camera. Pixel size is the key, not crop factor (althought the cropped sensor cameras tend to have smaller pixels which confuses things). And when you get a larger lens, the smaller pixels will give more reach with that lens too. But every time you increase focal length by adding TCs or use a camera with smaller pixels, there is less light per pixel, so noisier images unless you can lengthen exposure time (longer exposure time = more light).
Roger