>when developing apps 7 forms, Am I supposed to determine what screen mode is suitable for clients. I have an app looks good at 800x600 but does not fit all on screen at 640x480. How do I deal with other users who run my app and prefer 640x480?

If the app is for general consumption, you might be stuck with 640x480. In a corporate environment you can dictate the minimum resolution (at least have a chance at it). Whenever a new pc comes in here, I set it to 800x600.