You are quite right, some of the games listed in the OP will see little to no benefit with a quad core, these include ETW, NTW, COD
If you look at
this benchmark, the Q6600 is faster than the (faster clocked E6750) in Far Cry 2 (on of the games mentioned), due to the threaded nature of the game.
Dragon Age also sees
significant gains with more cores.
For the Sims 3 there is a
boost for going with a quad- but it isn't massive.
As for Mafia 2, I have no clue - I await some in-depth reviews after the release date.
But generally the trend is to use more cores in games. Many of the current crop of games are multi platform and often they are very similar to the XBox 360 versions. As the 360 has a tri-core CPU, these games are usually written with that in mind and often this leaks into the PC port.
As for "multi-core", I realise there is no industry standard for what it mean, so we are both right. But my definition is a CPU with 3 or more cores onboard (and not a many-core). I usually don't term 2 core CPUs as multicore as the term "dual-core" is so ubiquitous these days.
When I mentioned GTA4, I meant it as an example of a game that basically requires a quad core to play. Others include ARMAII and Battlefield Bad Company 2. Since the OP did say "etc etc" I don't think its out of order to suggest other games than the ones mentioned in the first post.