How to disable VGA on new server... reversibly?

You won't be able to disable the VGA graphics in the bios unless you have a secondary card pci/agp graphics card installed. You might see the option, but it won't let you do it unless there is some other output. If your bios is fancy it might have a serial output in which case you might be able to disable the VGA, just be ready with a serial console if you ever need a display.

The thing to do is change the maximum shared memory value to be as small as possible. 4M, 8M, whatever it will let you move it down to. Once you turn down that shared memory space it isn't going to take up much on your system. If 32M of ram is going to make a difference in the life of your server, buy another gig and make it really happy!