yes PCI 3.0 and PCI 2.0 are fully compatible: PCI Express 3.0
PCI Express 3.0 Base specification revision 3.0 was made available in November 2010, after multiple delays. In August 2007, PCI-SIG announced that PCI Express 3.0 would carry a bit rate of 8 gigatransfers per second (GT/s), and that it would be backward compatible with existing PCIe implementations.
i don't think of it as a marketing ploy, there are some insane quad SLI set ups that do benefit from the overhead bandwidth. but for the most part PCI 3.0 isn't necessary for excellent performance for most users.