If you have any issues try updating the BIOS. Sometimes 2.1 cards don't play nice at first in a 1.0 slot because of some of their extra commands. They are still 100% compatible with the slot though, so the BIOS update takes care of it
You have a better chance of a 2.0 working in a 1.0 slot. The 2.1 for some reason have issues with the 1.0. It really depends on how old your MOBO is. I have talked to people who pluged a 2.1 card in.
And they said it worked just fine and other people I have talked to said no it did not work. And like undsol said make sure you have been updating your bios and MOBO.
If you havn't bought the card maybe someone you know has a card with the 2.1 just to see if it will work. They are all suppose to be backwards compatible but the 2.1 has a little issue. Good luck and I hope it all works out with out any problems.