I am not sure about the nvidia framebuffer device, but vesa framebuffer definitely doesn't support widescreen resolutions.
Sax2 only configures the X server, which is independent from the framebuffer. The framebuffer is configured through kernel parameters from /boot/grub/menu.lst. Using the vesa framebuffer, you need to use the vga=0x<modenumber> parameter. To use another framebuffer, try video=<device>. Unfortunately I am on a Windows machine and can't look up the vesa table and exact syntax.
After changing the resoluton, you need to take care that the initrd contains a splash picture of the chosen resolution. To include the picture in initrd, run mkinitrd.