Q. The example you gave is not a standard screen size, can I use it?
A. Why not? There is NO reason whatsoever why you have to use 640x480, 800x600, or even 1024x768. The XFree86 servers let you configure your hardware with a lot of freedom. It usually takes two to three tries to come up the right one. The important thing to shoot for is high refresh rate with reasonable viewing area. not high resolution at the price of eye-tearing flicker!
Q. It this the only resolution given the 65Mhz dot clock and 55Khz HSF?
A. Absolutely not! You are encouraged to follow the general procedure and do some trial-and-error to come up a setting that's really to your liking. Experimenting with this can be lots of fun. Most settings may just give you nasty video hash, but in practice a modern multi-sync monitor is usually not damaged easily. Be sure though, that your monitor can support the frame rates of your mode before using it for longer times.
Beware fixed-frequency monitors! This kind of hacking around can damage them rather quickly. Be sure you use valid refresh rates for every experiment on them.
Q. You just mentioned two standard resolutions. In Xconfig, there are many standard resolutions available, can you tell me whether there's any point in tinkering with timings?
A. Absolutely! Take, for example, the "standard" 640x480 listed in the current Xconfig. It employs 25Mhz driving frequency, frame lengths are 800 and 525 => refresh rate ~ 59.5Hz. Not too bad. But 28Mhz is a commonly available driving frequency from many SVGA boards. If we use it to drive 640x480, following the procedure we discussed above, you would get frame lengths like 812 and 505. Now the refresh rate is raised to 68Hz, a quite significant improvement over the standard one.
Q. Can you summarize what we have discussed so far?
A. In a nutshell: