Well, it's an RPG and that type of game will always have a limited appeal. The best-case scenario for the genre is FFVII or FFX, where you have the genre's best game, published by the first-party console manufacturer and promoted as a system-seller. You also have the gen's best selling system and a large install base.
In the case of "X", it is the sequal of a critically-aclaimed game that has had outstanding word of mouth. It looks, on the surface, to be of even better quality. It's being published by the 1st party. It's initial debut was presented as a special, cornerstone title.
I think X will sell at least double of what Xenoblade did. Better consumer awareness, the great buzz of the previous game and a better release will ensure that. Can it do better? I think it could definitely do much better. I think it could be upwards of 5X...sales in excess of 3million is really where it should be. Final Fantasy 13 was generally disliked and their is an opening for a new flagship jrpg. The key to me is promotion. It needs to get a timely worldwide release. It needs serious 1st-party promotion by Nintendo. It needs to be treated as a key franchise and not an oversight. Additionally, there needs to be a large, receptive, install base. WiiU is the follow up to Wii, so even if it sells at 50% it should have a decent base. With DQX on WiiU and Mistwalker having stated a desire to bring more action RPG's to WiiU, the system could become the go-to system for the genre. If the market is there and if Nintendo treats X as a major title the game could be a system-seller to jrpg fans and sell 5X the original or more.