When Debian 6 (amd 64, testing) is loaded, before desktop appears, for a while, the screen is black and three strange green rows appear. Then, all is ok and I can see a normale desktop and start to use Debian.
So, an error in video config could be possible.
Linked to this fact (I think) , there is the impossibility to modify the resolution screen (that is fixed to 1024x768) and the refresh frequeency.
I've tried to find "xorg.conf" in order to modify it like varios guides suggest me, but this file is not present.

Any suggestions??

Current X does not use xorg.config by default, but rather probes hardward on every restart.

You can create one. This link tells you how:

http://wiki.debian.org/Xorg

Regarding the resolution, it could be using a generic driver. Try using the Debian system information tool to see whether your video card has been correctly identified.

When you say you see stuff before the desktop appears, is this before the login manager appears after logging in before the desktop environment/window manager appears? These may just be artifacts left over from the previous session. I do not use a graphical login and, I start X, it's common for me to see leftover artifacts from the previous session while the new one is loading. They are not so common when you use a graphical display manager only because the splash screen hides them.