[CentOS] X forwarding

How does one use X forwarding properly?
I got a server running CentOS 6.3 at runlevel 3 (no DE installed)
I try to access virt-manager via x-forwarding from another CentOS 6.3 
client in my network (runlevel 5 with default gnome)

on The server i changed my sshd_config:
X11Forwarding yes

i installed xauth on both of the machines.

Anything else there is to do?
When i try to:
ssh -X root at server virt-manager

I still get the following error message:
RuntimeError: could not open display

Which is in fact a message i get with every graphical programm i try to 
run through that. So I guess it's an error in my ssh configuration.
Any tips?