I feel that when dealing with 3D games, as long as there's no 2D-over/under-3D then scaling the screen res won't cause any problems as far as the looks are concerned. Therefore it's best to give the user the option of what screen res to play the game in. I usually give a choice of three: 640x480, 800x600, 1024x768, and with each one a choice of 16-bit or 32-bit colour mode. (Dithering is used in 16-bit)