This sounds like you only have one mode defined in your /etc/X11/XF86Config-4 files; it's probably 1280x1024. In your "Screen" section, Modes subsection, make sure that you have all the resolutions you want to play at listed. Generally, a line of
Modes "1600x1200" "1280x1024" "1024x768" "800x600" "640480"
will work fine. Please remember that the first listed mode will be the default desktop size, so if 1600x1200 is too big, just remove it from the list.
fat bastid: Make sure the line 'alias char-major-195 NVdriver' appears in /etc/modules.conf.
I hope this helps!