Most cards under windows can support 640 X 480 in 16-bit color...however, blitz has a couple of handy little built in functions for your game to deturmine the available graphics modes and if a 3D graphics driver is available...
I would first test if the graphics card can support 3D acceleration, then check if 640 X 480 X 16-bit mode is supported...If so then that is the initial graphics resolution I would start with, and set it in a window (not fullscreen)...after that you can allow the players to change the resolution basied on what the graphics card supports (take into account the memory requirements for all your textures...so you might not want the player to set a 1600 X 1200 X 32-bit mode if they only have 64MB of video memory and your game is going to need at least 40MB of it at 32-bit color depth)...once the player has changed the game resolution you can write those settings to a file and reapply them the next time the game is started (if no file exists then go with the windowed 640 X 480 16-bit setting)
I would first test if the graphics card can support 3D acceleration, then check if 640 X 480 X 16-bit mode is supported...If so then that is the initial graphics resolution I would start with, and set it in a window (not fullscreen)...after that you can allow the players to change the resolution basied on what the graphics card supports (take into account the memory requirements for all your textures...so you might not want the player to set a 1600 X 1200 X 32-bit mode if they only have 64MB of video memory and your game is going to need at least 40MB of it at 32-bit color depth)...once the player has changed the game resolution you can write those settings to a file and reapply them the next time the game is started (if no file exists then go with the windowed 640 X 480 16-bit setting)