you can edit XF86Config manually and insert the line for the 1600x1200 mode.
Now, I know, this is not recommended, and specifically prohibited in the XF86Config file, but this is the only way I figured out how to do it. My situtatiation was even worse, max resolution I could get via Yast2 or sax2 was 1152 * 864.
Generate the modeline with the xmode command and paste it into the XF86Config. Put it first, so it defaults to that resolution.
If anyone has any better suggestions, let me know. (sax2 recognizes my video card and monitor correctly. both can go to 16x12@85)