If you get only a black screen, odds are that X is running.
Have you checked your /var/log/Xorg.0.log for errors?
I'd start my debugging by ruling out Gnome all together..
Install TVM and xterm (and xclock, alternativly comment that row out)
# pacman -S xorg-twm xterm xorg-xclock
Then copy the contents from the default xinitrc to yor local .xinitrc
$ cat /etc/X11/xinit/xinitrc > ~/.xinitrc
If twm starts with some terminals (and xclock) the problem is in your gnome session.
At least, if it works, you can now install a web browser and debug from within twm..
Also, please post your /var/log/Xorg.0.log if it does not work..
There might be a Catalyst problem, try using the open source driver if this fails (after checking your Xorg.log)..