back FULLY on topic rather than only sharing the word "CPU"
A quad core would get much better performance than dual, even on older games that don't utilise all of the cores. All newer games now are multi-threaded (Can use more than one core/thread at once) so take advantage of the extra cores you get.
A dual core will play all games out now still, but for how much longer remains to be seen, they are getting outdated for gaming now most certainly. Although you were advised not to get a tri-core CPU, I have an AMD 720BE, which is a tri-core CPU, and am getting full settings on MW2, CoD4, CoD:WaW, CS:S, basically everything your bro plays and more.
If your brother is on a budget, go for AMD. They are much cheaper for the CPU, leaveing money for better motherboard, memory and graphics and will perfrom just as well in any game as an i5 or i7 will.
If he has a large budget, get an i5 or i7 because out of games things will load slightly faster, not massively mind, but a bit. An AMD quad core will still run everything very quickly, just not uite as qucik as intel will.
It is still more about your graphics card than processor though for gaming, you will get bottlenecked by your GPU long before your CPU on newer games (brought out in the last 5 years or so).