I'm at uni now, and all my Macs at home, so I can't power up my iMac for confirmation, but I'm fairly sure I remember that the iMac supports these resolutions,
ONLY at these refresh rates:
- 640x480 at 117 Hz
- 800x600 at 85 Hz
- 1024x768 at 75 Hz
Nothing more, nothing less. Even though the hardware could probably handle 640x480 at 67 Hz (the typical basic resolution supported by just about all beige Macs with built in video), it simply will not allow you to run the display at any combination of refresh rate/resolution other than the ones above. I've tried it and it doesn't work.
Connecting an external monitor to the iMac though, is a different story - you can connect any multisync Apple display, or with a VGA adaptor, just about any PC display, and run an external display at any resolution up to 1600x1200.