I know there are some people with Nvidia video cards (namely the 7900 series) that the fixed aspect ratio scaling option doesn't work. I have read it is a problem with some driver versions, or with some monitors. You will see a garbled screen or no change or the wrong change.
I have been playing around with my 7900gt trying to get the fixed aspect ratio scaling to work, and I came up with this.
I am using TweaksRUs driver version 93.71, and the classic control panel. I am not sure if any of that matters though.
Select the resolution you want. Then go to advanced timing. First under the timing standard select CVT standard and apply it. Go to flat panel scaling and select fixed aspect ratio scaling. Select the back-end active. The difference between it and the front-end active has to be the same as the difference between the aspect ratio of the resolution and aspect ratio of your monitor.
Example: .625(16/10) / .75(4/3) = .83333
.625(16/10) / .8(5/4) = .78125
So, for 1024x768 it would be 1024 / .83333 = 1228
Put the 1228 in to the back-end active and hit apply. If all goes well it should have black bars on the sides. If the screen goes blank or you get some kind of error back it off a little bit to say 1225. If I went to high the monitor would say no signal in put, and I would have to restart the computer.
Screen shot:
1440x900:
1024x768 pillarboxed
This has worked in the few games I have tried, if you select the resolution you want in the desktop before you start the game.
This worked for me anyway. I would be interested to know if this works for anyone else.