Is this coming out for PC at the same time as 360? If so I would bet for two reasons (one good and one bad) it will do worse then expected. On the good side a lot of people who have both a 360 and gaming quality PC will get it on the PC. As Fallout 3, Oblivion, Mass Effect and many more show if you have the choice things will undoubtedly look and play FAR better on a PC unless your PC is a dog (using anything made by AMD for instance). I don't think anyone with both a very good (capable of very high settings) computer would argue against getting games that are on both for the PC, but there might be ONE guy out there somewhere.
The bad reason is a lot of the people who aren't motivated enough to mod chip the Xbox 360 (and might actually buy it for 360) are certainly capable enough to just DL it from Pirates bay. I like to pretend that isn't most of the community, but I bet its at least 30% of potential customers. I've pirated music, some movies (ones not worth 20 bucks to see in theatres or buy on DVD) and especially TV shows (which are aired free anyway) but I would never pirate a game; most of the non casual community doesn't seem to agree however.
My guess is a few hundred thousand unless it pulls something near straight 10s. I hope its good, the last weird adventure type game I've played that really impressed me was Indigo Prophecy which is what, 6 years old now?