PCI Express 2 and 3 are forward and backward compatible, but you apparently have a 1st Gen PCI Express. Those should be compatible (and usually are), check the detailed card specs or ask the manufacturer support to be 100% sure. Your performance may suffer a little though as the speed of PCI Express Gen 1 is a bit slower, although you do have an x16 slot so it should not be too bad.
edit: Zotac specifically notes that their models are all backward compatible to 1.1 slots.