In X.org (and for 4.0.0 and later versions of the now-obsolete XFree86) you no longer have to generate modelines at all under most circumstances. Instead they are computed internally by the server at startup time, based on the resolution you specify in the the monitor capabilities your X server gets via an EDID query to the monitor (and the Modes part of the Screen section part of your X configuration file, if you have one).
To change your screen resolution and color depth, simply edit or create a Display section describing it. Here is a sample Screen description from the X configuration file of my laptop:
Section "Screen" Identifier "Screen0" Device "ATI Rage Mobility" Monitor "Monitor0" DefaultDepth 16 Subsection "Display" Depth 16 Modes "1024x768" EndSubsection EndSection
All you will usually need to do is change the numbers in the Modes entry. X will do the rest. If you specify an impossible resolution, it will fall back to the closest approximation that the EDID data from the monitor says it can support.
Therefore, the information in the remainder of this HOWTO is useful only if (a) you have an old, pre-EDID monitor, or (b) your graphics-card driver doesn't support querying the monitor, or (c) you are running a very old version of X (in which case, you should fix your problem by upgrading), or (d) your monitor/card combination operates outside the range over which X has canned modelines.