SLI question.

Hello, i m new on this sli things. My question is:
i have a nvidia 7600gt and a 7950 gt, can they be connected together is a sli mode or they have to be equal graphic cards.
Many thanks
i have a nvidia 7600gt and a 7950 gt, can they be connected together is a sli mode or they have to be equal graphic cards.
Many thanks