SLI is multiple cards working as one effectively doubling the performance but they have to be hooked to each other inside the machine (SLI) or hooked up with an external cable (crossfire) it's not just that. The cards also have to have a supporting chipset and a driver that supports it. There has been scattered reports of some people getting the 7300Gt's to work is a software supported SLI with a driver hack. The 7300's are 1 example of an Nvidia card that does not need the physical connection to each other.
Just because the Mac Pro has 4 slots does not mean it should support SLI. The Mac Pro uses an intel 5000x chipset that was not designed to support SLI the 5000x was designed to be a workstation/server chipset.