You might try this: Design two card sets of images, one for 800x600, the other for 640x480. As the others have already pointed out, these two resolutions are pretty much the standard ones for the overwhelming majority of users out there. Just be sure to use the command to be certain whether a certain resolution mode exist on their graphics card.
GfxModeExists (width,height,depth)
GfxModeExists (width,height,depth)