Re: About to purchase an X-Box 360....
Mass Effect is hands down the best game for the 360, and I believe out of them all (I also love Bioware, too). That being said, I hated Oblivion (partly because I loved Morrowind, the game before it, and thought Oblivion was an awful sequel), and could never really get into Fallout 3. Those two games are better for the PC because of all the mods that the community has made.
No, Mass Effect is a space epic action-RPG not unlike Star Wars in many respects (and in many ways, much, much better). That ad is for Dead Space, the space horror game.
Nah, Rockband 2 is better. Better songs with DLC, more refined gameplay, and RB1 songs, as well as song imports.
That being said, here's some other fun games:
The Halo Series (even Wars)
The Orange Box
Call of Duty 4: Modern Warfare
Gears of War (both)
Mirror's Edge is also really good but it should be a rental.