I tried a second time by first stopping lightdm and then running startx from the console:

antti@beasty:~$ sudo stop lightdm
lightdm stop/waiting
antti@beasty:~$ sudo startx


 Now trying to play a video with mplayer X crashes in another place:

Program received signal SIGSEGV, Segmentation fault.
0x00007fd23fd148f6 in X740XvPutImage ()
   from /usr/lib/x86_64-linux-gnu/xorg/extra-modules/extra-modules.dpkg-tmp/modules/amdxmm.so
(gdb) bt
#0  0x00007fd23fd148f6 in X740XvPutImage ()
   from /usr/lib/x86_64-linux-gnu/xorg/extra-modules/extra-modules.dpkg-tmp/modules/amdxmm.so
#1  0x00007fd247e8ebae in ?? ()
#2  0x00007fd244e99cb2 in ?? ()
   from /usr/lib/xorg/modules/extensions/libextmod.so
#3  0x00007fd247e36721 in ?? ()
#4  0x00007fd247e2564a in ?? ()
#5  0x00007fd245fa576d in __libc_start_main (main=0x7fd247e252b0, argc=6, 
    ubp_av=0x7ffffdcf05c8, init=<optimized out>, fini=<optimized out>, 
    rtld_fini=<optimized out>, stack_end=0x7ffffdcf05b8) at libc-start.c:226
#6  0x00007fd247e2593d in _start ()


And here's the output from startx:

X.Org X Server 1.11.3
Release Date: 2011-12-16
X Protocol Version 11, Revision 0
Build Operating System: Linux 2.6.24-30-server x86_64 Ubuntu
Current Operating System: Linux beasty 3.2.0-18-generic #28-Ubuntu SMP Fri Mar 2 20:02:50 UTC 2012 x86_64
Kernel command line: BOOT_IMAGE=/boot/vmlinuz-3.2.0-18-generic root=UUID=a8bcb914-d443-415c-bc5c-395358d8e52a ro crashkernel=384M-2G:64M,2G-:128M quiet splash vt.handoff=7
Build Date: 08 March 2012  12:01:51AM
xorg-server 2:1.11.4-0ubuntu5 (For technical support please see http://www.ubuntu.com/support) 
Current version of pixman: 0.24.4
	Before reporting problems, check http://wiki.x.org
	to make sure that you have the latest version.
Markers: (--) probed, (**) from config file, (==) default setting,
	(++) from command line, (!!) notice, (II) informational,
	(WW) warning, (EE) error, (NI) not implemented, (??) unknown.
(==) Log file: "/var/log/Xorg.0.log", Time: Sun Mar 11 20:42:21 2012
(==) Using config file: "/etc/X11/xorg.conf"
(==) Using system config directory "/usr/share/X11/xorg.conf.d"
(WW) fglrx: No matching Device section for instance (BusID PCI:0@4:0:1) found
[-     XMM_GLX] [I ]glesxXvInit Configureable RGBOutputColorRange

Backtrace:
0: /usr/bin/X (xorg_backtrace+0x26) [0x7fd247f70206]
1: /usr/bin/X (0x7fd247de8000+0x18c0aa) [0x7fd247f740aa]
2: /lib/x86_64-linux-gnu/libpthread.so.0 (0x7fd24710e000+0xfcb0) [0x7fd24711dcb0]
3: /usr/lib/x86_64-linux-gnu/xorg/extra-modules/extra-modules.dpkg-tmp/modules/amdxmm.so (X740XvPutImage+0x146) [0x7fd23fd148f6]
4: /usr/bin/X (0x7fd247de8000+0xa6bae) [0x7fd247e8ebae]
5: /usr/lib/xorg/modules/extensions/libextmod.so (0x7fd244e8b000+0xecb2) [0x7fd244e99cb2]
6: /usr/bin/X (0x7fd247de8000+0x4e721) [0x7fd247e36721]
7: /usr/bin/X (0x7fd247de8000+0x3d64a) [0x7fd247e2564a]
8: /lib/x86_64-linux-gnu/libc.so.6 (__libc_start_main+0xed) [0x7fd245fa576d]
9: /usr/bin/X (0x7fd247de8000+0x3d93d) [0x7fd247e2593d]
Segmentation fault at address 0xa4

Caught signal 11 (Segmentation fault). Server aborting

Please consult the The X.Org Foundation support 
	 at http://wiki.x.org
 for help. 
Please also check the log file at "/var/log/Xorg.0.log" for additional information.

 ddxSigGiveUp: Closing log
Server terminated with error (1). Closing log file.
xinit: connection to X server lost