I did search a bit but am still confused, so sorry if this was answered somewhere else.
Please just put yes or no to make this easier
1) I'm pretty sure PCI Express 2.1 x16 will work on a motherboard that supports PCI Express 2.0 x16 but just to make sure [YES/NO]
2) Will PCI Express 3.0 x16 work on a motherboard that supports PCI Express 2.0 x16 (when it comes out) [YES/NO]
3) Will PCI Express 2.0 x16 or 2.1 work on a motherboard that supports just PCI Express [YES/NO]
2) Probably; they are trying to keep PCI-E fully backward compatable, and delayed PCI-E 3.0 for just that purpose
3) Sometimes [mostly?]; most cards can still run in a standard PCI-E x16 slot, but some cards/motherboards may have issues.