If you think you can tell how many cores a game is programmed for looking at how many cores have 'activity' on them while gaming ... you are entirely mistaken.
The way that you tell how many cores/threads a game is coded for is as follows:
1) Run the game with all the settings at their very lowest. The idea being you want to create an entirely CPU-dependent scenario.
2) Go into task manager, and choose to option to show all activity on ONE graph.
3) Observe at what level the CPU usage maxes out while gaming.
4) Take that percentage,and multiple that by how many cores/threads you have.
The resulting calculation will tell you how many cores the game is coded for. Having more cores/thread than what the game is coded for will not provide any performance boost, except for the degree to which you have extra resources for background windows tasks.
So, for example, if you see that total CPU usage maxes at 50%, and you have a quad core w/o HT, then the game is coded for 50% x 4 = 2 cores.
The only game I've ever seen use more than 50% on my rig (4 cores w/HT) is Prototype. That game will most definitely utilize > 4 cores/threads. There may be others, but I've not encountered them. BFBC2 can use up to 4 cores/threads, not 8 as was stated above.