Russ Brandon, the team CEO, will also be named the team president, per various sources (Alex Marvez, Jason La Canfora, Jerry Sullivan). That move was made to give Brandon final authority over the team's football operation, which previously resided with team owner Ralph Wilson. In the words of WGR 550's Howard Simon, "Ralph's out" - meaning that Brandon is now the de facto owner of the Buffalo Bills.
Put more simply: Ralph owns the team, but he's removing himself from the decision-making process. Brandon now runs the whole ship, and GM Buddy Nix - who will remain with the organization - will report to Brandon instead of Wilson.
The other big piece of news, per La Canfora, Simon, Adam Schefter and more sources, is that not only is Nix staying, but a transition plan to Assistant GM Doug Whaley has also been put in place. Nix and Whaley will work to hire the successor to Chan Gailey at head coach, and Brandon will surely be involved. Whaley was hired in 2010 with this plan obviously in mind; now the timeline may be moved up.
This is huge news. We'll continue to update as the situation unfolds.