I think the placement of the image on the screen is dependant on the sync freqs. Resolutions can run in range of freqs. If it is a modern monitor, it should remeber image size and placement on the screen for the particular res. If the freqs change slightly, we are talking tenths of a kHz for the horizontal, it interprets it as a "different" res. I run my Dell P991s at 106.7kHz hor and 85Hz vert at 1600x1200. Those are the freqs that I got from my monitors' OSDs while in Win2K. So then in my X config in linux I put those EXACT freqs for the syncs.
I dunno I could be wrong, but that is my theory and it worked with my particular setup. Try it, you never know.
BDOG