Well, there's only one 50" 1080p plasma - the Pioneer Elite. That isn't carried by Circuit City or Best Buy, but Magnolias inside Best Buys have them.
The XA2 is the high end model, and I doubt if CC or the "core" BB stores will carry them. They will likely be in Magnolia, though.
So it sounds like, that among the two major chains, BB with a Magnolia will be your best chance to see that combination.
I love HD...and I hated that Anti-Consumer format war.
Now that we can move on... Blu-ray > Digital Downloads!