the computers all with graphic card, I only want to use it for mining during idle time.
how much BTC is possilble to generate per day with this computing capability?
That all depends on what the graphics card is.
A decent overclocked card can currently generate about 1 BTC per day. You could probably expect that to halve in a week's time, and then halve again, if the current difficulty increase trend continues.
ATI cards are the best for this, especially the 5850.
My guess is your machines are using the video chipset on the board. If that is the case, then it may not be worthwhile using them as miners. Then again you never know. You need to find out what the graphics card/chipset is.
edit: I just read that your machines are for gaming... must have decent graphics cards in.. what be they ?