There could be one or two regular PCI slots with 2 PCIe slots and 2 PCIx16 slots.
The PCIX16 slots will be the extra long ones, PCIe slots look really short and then theres the plain PCI slots and theres something called PCI4, dont know what the heck that is for. There are currently some new boards that can run dual SLI on one card, forget the brand though.
On this pic, the two on the left are the regular PCI slots, the long ones are the PCIx16, one PCIx1 and one PCIx4.
The difference is the board must be SLI capable and have those 2 extra long looking PCIx16 slots.