I wouldn't go and replace that just yet. For starters clear and make sure it comes back...

It could be caused by a misfire, could be air leaking into the system (either side..), fuel pressure could be off, compression could be off, and finally it could be the cat.

I believe this code gets set when the reading on the front is very similar to the reading on the rear O2 sensor. When the Cat is not working they'll be the same, but before you spend a grand or you might as well make sure that's the problem..

I don't know if you've cleared yet or not, but clearing and making sure it comes back, as well as making sure the car runs all the tests before you replace is important. Misfiring would recognize as a separate code, but uneven compression wouldn't (i don't think at least..).