So I installed Debian with netinstall. Installed gnome. My graphic card is radeon 9550.
I read that I have to install firmware-linux-nonfree. I did it and almost everything ran ok. But I downloaded a 3D game to test the graphics and it didn't render too good. This is where I screwed up.

So I installed fglrx-driver. And this screwed everything. With this installed I can run no game, the window just doesn't appear. If I remove it, games will launch, but render very very sloooow. I tried reinstalling linux-firmware-nonfree. But same thing. Exactly as it was when I just installed gnome.

Fglrx installed does better in benchmark. But it just doesn't allow any game to run. It's strange. Also I installed compiz after that to se how it works, it return fatal error when I enable it.
Guess if I don't fix it I will have to reinstall debian.

The Radeon 9550 has no support from AMD/ATI for quite some time now. Your only option is to use the free radeon drivers, which should work pretty well on this card.
I wonder how you got the fglrx driver working at all, it normally should refuse to work at all with unsupported cards.
I would recommend to remove the fglrx-driver and the file /etc/X11/xorg.conf and see if you get your system back in a working state.

Yea I see it's refusing because games wouldn't even launch.
What do you mean by free radeon drivers? The xserver drivers are already installed. Exact package name. Before I installed fglrx-driver it weren't that bad. Then firmware-linux-nonfree fixed things.
I removed fglrxand, re-installed firmware-linux-nonfree and libgl1-mesa-dri and still I'm not able to play simple 2d game. (very low fps). Things are just exactly as were before I installed firmware-linux-nonfree

The free radeon driver is the one that cames installed by default (xserver-xorg-video-radeon) and that needs the unfree firmware for proper 3D acceleration.

How exactly did you install the fglrx drivers and how did you remove them? The fglrx driver replaces several libraries with its own versions, so if there went anything wrong during the install or uninstall phase your system may be messed up. If that is the case and you don't have spent much time in configuring your system by now a clean re-install of Debian may be the fastest option to solve your issues.
Also, keep in mind that Debian 7 (Wheezy) is already in its frozen state, which means that it will be released in the next months. The radeon driver in that version has some great improvements, regarding features and rendering speed, so may be trying that version may be an option to get better performance.

I installed it with apt-get, removed with apt-get. And autoremoved what remained.

The fglrx driver needs the radeon driver to be blacklisted for proper functioning. Please check the configuration files in /etc/modprobe.d if the radeon driver is still blacklisted. This would explain the slow 2D performance.

Yes there is fglrx.conf with blacklist radeon in it. Now everything should work. Thanks.