Video modes for X?

["John Gay" <John_Gay@eur.3com.com>]

I've got Debian 2.1 working on my PII 350 with an AGP ATI 3D Rage IIc Video
card, but I only configured one resolution, 1024 X 768. I'd like to add more,
for things like mpegtv which needs to be able to switch resolutions to enable
full-screen video. I know where the file is that needs to be changed, I even
know where in the file to make the changes, But I'm a little wary to make any
changes in case I break my working system. I've got the manual for my monitor
and it lists all the resolutions with the various sync rates and frequencies,
but these don't match up with the info in XF86Config. I know the line is
something like;

"1024 X 768"  35.31 1024 1027 1028 1031 768 771 775 781, I'm not at the PC, but
you get the idea.
     ^           ^         ^        ^                               ^      ^
^
Resolution    clock         Horiz freq's                  Vert freq's

These don't match my manual. My manual lists the different resolutions with the
clock freq, then the horiz and vert freq's are listed by period. Is there a
simple formula to translate these periods to the settings used by X, or does the
Video card have some influence on these setting as well. I used XF86Setup to set
up my X server. Is there a simple way to back up the settings for X? What files
should I save? My system automatically boots into X using xdm. If I trash my X
settings, can I get into a terminal to move the back up files to recover from
serious failures? I meant to bring in the manual for my monitor to copy the info
into this mail, but I left it at home. Would this info be useful to someone to
suggest settings for X? I can bring it in tomorrow.

Thanks for all the help and I look forward to getting different video
resolutions working so I can enjoy full-screen Video.

Cheers,

     John Gay