Not sure yet. I'll check out reviews and possibly pirate it first to see if it's worth buying. Most likely, I'll end up waiting for a sale. I'll be plenty busy with Borderlands 2, Diablo 3 and DCU Online. And I expect sales of some Source games around the time the Linux Steam client goes live, which will keep me further preoccupied. Combined with the ridiculous number of games I own which I still haven't played (****ing Steam sales), I'm in no rush to buy Dark Souls.
For now it is undecided. I will buy it if the extra content is not released as DLC for the console versions AND it is Games For Windows Live with a new set of Achievements. Dark Souls is my proudest 1000/1000 on my GamerTag and it could only be better if it was doubled.
If, however, the stuff is released as DLC on consoles OR it's something other than Games For Windows Live, then I'll most likely pass.
To be honest, I don't know if my computers could handle it anyway. Even if it can and I end up buying it, it probably won't be until it's down to about half price. --- Like the "100,000 Strong for Bringing Back Mega Man Legends 3" fan page on Facebook! (http://tinyurl.com/3gj2r6g)