Help.........Alot.
Bioshock is now the X360 game TO GET. Even beyond Gears of War as a #2nd purchase game.
I would argue that Bioshock caters to a similar crowd as Halo, so I think it'll benefit quite a bit. Every game, imo, will benefit, because the price drop had a positive effect on all games.
However, in the beginning of the Halo3 launch, I don't know if Bioshock will be the top-game to get an increase. Simply because Gears, RSV, Dead Rising, Saints Row, and Oblivion are all Platinum or GOTY editions...Which I would think with all these people putting down $400 for new X360s, might want to go the cheaper route and get a cheaper game.
This trend has been evidenced strongly even in Japan for the X360 - when the X360 dropped Trusty Bell, every game from Gears of War to Viva Pinata and Blue Dragon had major 30% increases....When Halo3 manages to move 250,000 systems over normal sales in a matter of 2-3 weeks (or more systems), these people WILL buy ALOT of games for Christmas.
So during Christmas, we have the following 'big games' for people to buy:
Halo3 - Most will already own, but should sell 100-250k/wk
Call of Duty 4 - Post Halo3 FPS that's must-have
Mass Effect - WRPG GOTY.
Bioshock - Duh...Top 10 game of all time.
Project Gotham 4 - XBox's top racer w/ actual userbase now.
NFS: Pro-Street - NFS series always sells well
+ Budget titles.
Those top 6 games - which ones will do the best? Most likely H3, CoD4, ME and Bioshock will all manage around 100k/wk in December. That could boost Bioshock to 1.5m or even 2m by the end of the year...Simply because between H3 launch and the end of December, your going to have well over 2m new X360 users, and probably a 4.0 software attach ratio.