No then Yes.
The original battery cover won't fit on the extended battery.
Almost all cases will fit over the Droid X with extended battery. The biggest thing is if the case is soft and pliable. I don't have the Active but I do have the Limited edition Active X and that DOES fit over the extended battery.
If you get a case that is too rigid and won't fit right, then maybe you could just leave the battery door off the Droid X, then it will fit.
I think you should be fine, every case I have tried fits the extended battery even if the manufacture says it won't.