The 7600 GS is not a good card price/performance wise.
I suppose if u already have the 7600 GS, u could buy another and have them work together in SLi, but its a waste of money, because for the price of 2 of these u could have gotten a much better card, and then bought another after some time has passes - thats the whole point of SLi, buying a single card that costs like 2, and then another identical card after a while.
You can buy an extra PCI, IDE connecter and have as many IDE slots as u want.