since you were there you could have just checked the wikipedia page for pci-e 3.0 but anyways.
it has twice the bandwidth per lane than pci-e 2.0 and like all other pci-e slots they are forwards and backwards compatible.
so yes your old card will work with it. and new cards that are pci-e 3.0 will work on older versions of the slot.
Actually PCI-E 3.0 is still a proposal as there is no complaint hardware. Yes, Intel has incorporated PCI-E 3.0 capability into its Sandybridge E platform, but it has refrained from calling it PCI-E 3;.0 until the responding hardware (read video cards) arrive. There is speculation that the delay in the release of the next generation of video cards is the result of trying to be PCI-E 3.0 compliant.
PCI-E 3.0 like USB 3.0 lacks the hardware to take advantage of the bandwidth. SATA 3.0's bandwidth is used only by SSDs currently. All are future standards, but lack the hardware to fully utilize their advantages.