There are always some exceptions, these are few and far between. Sometimes a certain motherboard wont like running X or Y GPU but generally they are all backwards compatible.
You shouldn't get issues with a 2.0 slot, it tends to be 1.1 and lower where the issues start. As I said even then very few and far between.
I just mention it so that you are aware there is not 100% compatibility right across the board. Always check if you are unsure.