i recently bought a 17" flat screen monitor so i can use my computer in a isolated sound booth via mini-dvi to VGA adapter. When i plug in the adapter, the screen size changes on my computer (ie. the screen shrinks, leaving black spaces to the right and left). Finding this an enormous waste of screen space, i keep unpluging and repluging the mini dvi adapter several times a day as i go back and forth. I would love to keep it plugged in all day. Is that possible? Also, am i creating extra stress on the computer by doing this?
The caveat of mirroring your desktop is that both displays must necessarily be using the same resolution. The only way around this black border issue is really to get a monitor with the same aspect ratio as the 20" display on your iMac. Mirroring your desktop doesn't put as much additional load on your machine as desktop spanning does, and even then it's not much - it was designed to handle that sort of load. I wouldn't worry about it.
I keep my resolution on 1680X1050 with an aspect ratio of 16:10. If I purchase a 19" monitor with that ratio (the max resolution is 1440X900), can i assume that the resolution on my iMac will switch to 1440X900 when the external monitor is plugged in?
Yes, exactly - for as long as you're configured to mirror your desktop. The picture will be 'blurred', but the black borders shouldn't be an issue, I don't think.
Initially, I was worried that it might turn out to be a case of a 1440x900 desktop
framed in the 1680x1050 pixel space, but your iMac seems to be stretching the image to fill the limiting dimension. So, I'm assuming it'll be the same with a 19" monitor running at 1440x900. I hope that makes sense.