If you enter [CTRL]-[ALT]-[F2], can you get into a virtual terminal? If so, then I'd recommend running yast2 in the text mode.
The first thing I would do is run the "Community Repositories" module and add a repository for ATI. The run the Software Installation module and from the ATI repository, install the ati proprietory drivers for your graphics card.
After that is done, run "sax2".
As an alternative, you could try using the generic "vesa" driver in your /etc/X11/xorg.conf config file. This may allow you to run graphically. I don't have an ATI chip but I have had a similar problem initially with my NVidia card. The video was twice as high as it should have been and the bottom half was cut off. Installing the nvidia driver fixed the problem. (nvidia vs nv)
Something else that can cause a problem is if you have both a built in graphics driver on the MB and a second graphics card. If that is the case, you may need to add busID statements in your /etc/X11/xorg.conf file.
From the xorg.conf manpage:
This specifies the bus location of the graphics card. For PCI/AGP cards, the
bus-id string has the form PCI:bus:device:function (e.g., “PCI:1:0:0” might be
appropriate for an AGP card). This field is usually optional in single-head
configurations when using the primary graphics card. In multi-head configura‐
tions, or when using a secondary graphics card in a single-head configuration,
this entry is mandatory. Its main purpose is to make an unambiguous connection
between the device section and the hardware it is representing. This informa‐
tion can usually be found by running the Xorg server with the -scanpci command
If you look at the output of /sbin/lspic, you can get the bus-id to use. Example from my laptop:
01:00.0 VGA compatible controller: nVidia Corporation NV17 [GeForce4 440 Go 64M] (rev a3)
So in my "Device" section of /etc/X11/xorg.conf, I could have a BusID entry:
BoardName "GeForce4 440 Go 64M"
This will prevent the 'Section "Device"' for one graphics controller from being confused for the other. The 'Section "Screen"' section will contain a line near the bottom identifying which graphics device should be used:
Modes "1280x800" "1280x768" "1024x768" "1280x600" "1024x600" "800x600" "768x576" "640x4
This will help the X server identify which controller is which.