So I just assembled my new PC today. It's awesome. Liquid smooth graphics and everything, except an annoying problem..
 
I have a 20.1" widescreen LCD monitor with an aspect ratio of 16:10. This is hooked up to a HIS Radeon HD2900 XT 512MB GDDR3 video card via a DVI-D cable. In games like Halo, which max out at 1280x1024, the image is distorted, stretched horizontally. I would prefer that the image is displayed centered in the normal 4:3 aspect with black bars at the edges. I enabled the "Use centered timings" option in the Catalyst Control Center, but it makes no difference, except on a 1154 by something resolution, but even in that the image is stretched horizontally.
 
I'm pretty sure this is a common problem with a common fix, so please help. Thanks!  

Happens to me with an X800 too so I am interested in seeing how to over come this too.

if you have any settings monitor side you will need to adjust those too.  that is how it is with me.  i need to telll it to stretch, maintain aspect ration or use 1:1pixles.
 
  however, funny thing is i have tried games at 12 x 10 full panel like oblivon and it looks fine.  although i run games at 19 x 12 i like to see how they look at lower res.  although not as nice i find that most games look fine at lower ones especially with tons of AA enabled.

 
 
Centering the image on a monitor is usually dependent on the monitor, not the video card.  The "use centered timings options" you are referring to is for refresh timings or something along those lines.
 
So in short the monitor/HD TV/projector you are using needs to centre the image, the video card doesnt know what the monitor is doing, it just outputs a signal.  
 
if your monitor has screen adjustment control you can try using those to shrink the image horizontally, but usually on LCDs u can only move the image around and not shrink it like on CRTs.
 
another solution would be to get the game you are having issues with to run in a 16:10 ratio.  im not sure if halo for pc can be hacked to make it run at a widescreen resolution, you would have to google that.
 
Many bigger screens usually have options as a previous poster mentioned to centre the image, it all depends on your screen, i have a 19" 16:10 screen and it has no such options so if i want to a play a game that only outputs 4:3 then im stuck with the streching.

not quite true.  the gfx card is the one that decides which resolution it is.  yes, a monitor can have scaling options but so does the gfx card.
 
  centred timings means it will centre the desktop regardless of if it is the same size as the panel.  or so the CCC says.
 
  however, i believe it is the monitor that has the final say in it as it were but the gfx card and the monitor should be set the same to ensure the best results.

 
yea I've seen the options from the gfx card side on laptops mostly, my laptop has that function and it seems to work most of the time by setting it in the driver.
 
hey cefarix why not just play halo on xbox