800x600 is not a Macintosh resolution. However, some Macintosh computers can support this resolution when used with the appropriate monitor. Unfotunately, computers using System 7.5.1 and higher do not support non-Macintosh "hidden" resolutions, such as 800x600 and 640x400.
If your computer and monitor can both support the non-Macintosh resolution you are looking for, and you do not have System 7.5.1 or higher, you may be able to use the following procedure to select a non-Macintosh resolution.
Step 1
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Choose Control Panels from the Apple Menu.
Step 2
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Open the Monitors control panel.
Step 3
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Hold down the Option key while clicking the 'Options' button in the Monitor control panel.
Step 3
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Choose the desired resolution, then click OK.