Question about "Can't open display: localhost:0.0" RHEL5

I'm trying to install Oracle 11g on RHEL5. But after I add user  oracle, it
cannot log in XWindow. I can log in as root.
I tried "xhost +" or "xhost +si:localuser:oracle", but it seems like:

[root@ern pam.d]# xhost +
access control disabled, clients can connect from any host
[root@ern pam.d]# xhost
access control disabled, clients can connect from any host
INET:ern
SI:localuser:root
[root@ern pam.d]# su - oracle
[oracle@ern ~]$ xclock
Error: Can't open display: localhost:0.0
[oracle@ern ~]$

And after restarting the machine, it didn't work either.
I stopped SELinux through GUI tools. Can anyone help me out?
Thanks in advance.

-- 
Yao Yongping
Learning Oracle, UNIX/Linux...
Love Reading, Classical Music, Philosophy, Economics etc.
Blog:  http://spaces.msn.com/yaoyp/

Other related posts: