does pci express 16x work with pci express 2.0?

Yes, it'll work.

If it's PCIeX16 2.0 yes. In a nut shell 2.0 is just telling you how fast that hardware can transfer data. If you put a 2.0 vidoe card on a motherboard that supports 1.0 it will work but will only transfer data at the 1.0 speeds. You can also put a 3.0 on a 1.0 pcie. The difference is a motherboard with 1.0 transfer rate and a 1.0 pcie video card will be slower than if both were 2.0 or 3.0. For more detail google "PCIeX16 1.0 vs 2.0" and you can read all the details about transfer speeds and what not.

Yes, most are compatible.