Team of dedicated people:
programmers, artists (depending on the genre, I guess 3D modellers and concept artists at least ...), game designers, musicians.
You can develop it in the Microsoft world or open source world, either way you can achieve portability for many operating systems. The MS tools are a bit more expensive (Express isn't for commercial use), while open source tools are free (g++, code blocks, ...).
Having an office helps, because people concentrate on development, if they work at home, they tend to be slower ...
Tip: Having the main plot outlined in a document with decent formating and having some quality concept art helps attract the investors