Now, based on what you learned above, some questions:
what graphic driver are you using now to get 800x600 ? Please check !
does your openSUSE have an /etc/X11/xorg.conf configuration file ? Please check !
based on what you read above in the 1st link, do you need guidance on how to run “sax2” to try either the “fbdev” or “vesa” or “nv” driver and better tune your resolution ? Or would you like to be walked through this ?
In the case of using the proprietary “nvidia” driver, there are MANY ways to do this, and I can walk you though the method I use (which is called the “hardway”, although it is NOT hard) if you wish.