you can. all pci-e cards are backwards are forwards compatible, but they may be limited by the bandwith of the slot.
*May* being the key word. A 16x PCIe 1.x slot can transfer 4 GB/s in each direction; that is plenty of bandwidth for current gen and most likely even next gen cards. PCIe 2.0 is handy when you're working with 8x slots, since you can get the 8GB/s of bandwidth with only 8 lanes