Far Cry 2 uses a proprietary engine that was built from the ground up for the game. The developers told me that about 175 people worked on creating the engine and then the game for the PC. Currently that team is working to build in DX10 support, though it wasn't ready when I had a chance to look at it.
Because the engine was built to work on a PC, bringing the game, with all of its bells and whistles, over to the Xbox 360 was a relatively easy affair, the developers told me. Afterall, they said, the Xbox 360 is essentially a computer. In fact they only have three people working on the team that is porting the game over.
The Playstation 3? Not quite as simple an affair. They have a team of 14 working on that port, mostly because of the "difficulties" of working with the Playstation 3's unique architecture.