I personally use a BenQ FP241VW (24", 1920x1200). It has 3 modes: fit to screen (which stretches the image to fill the panel), keep aspect ratio (which will keep the aspect ratio, so if you use it with resolution like 640x480 it will 'touch' the frame at the top and bottom and you will have black bars on the right and left side), 1:1 mapping (which will display the image in the centre - good for high, non native resolutions like 1400x1050).
Playing at 640x480, 800x600, 1024x768 using a Radeon 9800 Pro connected through DVI works without problems. On some odd resolutions (like 512x384, 640x400) the Radeon will send to the monitor image at 1920x1200 with the image in the center. 320x200, 320x240, 400x300 - the ATI squeezes the image and sends it to the monitor at 1920x1200, in the middle surrounded by black borders. (I believe it's the Radeon which converts it to 1920x1200, because using the aspect ration controls have no effect). 640x350 works nice with widescreen (at least in GLQuake).
Through D-Sub (analogue) 320x200 and 320x240 work without problems, 400x300 is stretched verticaly, 512x384 looks messy. 640x350, 640x400 (which didn't work well through DVI), 640x480 work OK (scaled keeping aspect ratio).
Haven't tested text modes (but you can see my BIOS screenshots from a 386 box with Oak ISA VGA here: ), but most LCD's I've seen mess them up, especially through the D-Sub cable.
Also modern VGAs have aspect ratio controls for DVI. My Thinkpad T61p has a 1920x1200 widescreen display which normally will stretch everything to fill the screen, but nVidia Quadro allows me to select between: fill screen, keep aspect ratio, (no 1:1 mapping as far as I know, but if connected to an external EIZO monitor I can also select this option). I tested 640x480, 800x600, 1024x768 on it and they work well. (except the image is blurry, but this effect will be on every LCD if you use it with a non native resolution).
As you can see it's best to do some testing not only with the choosen LCD but also VGA and different cables. Or at least ask people who already have the LCD you want to buy on some hardware forums. But I'm pretty sure 640x480 and higher resolutions will work ok with most panels.