A multi core processor is nothing more than putting more cpu's into one package.
if a multicore cpu is rated at 2.4 GHz, every single core runs at 2.4 GHz.
If a game or app is multithreaded, it will be able to use more than one of those cores at a time.
Do not assume that the clockspeed of a quad core 2.4 GHz cpu is 4 x 2.4 GHz (9.6 GHz). Its just 2.4 GHz.
The cores in a multi core cpu cant magically share the load if a program is not multi threaded. they have to be explicitly told to do work, there is no such thing as reverse hyperthreading.
Windows itself will use lots and lots of threads, so even if a game isn't multi threaded, Windows will be able to use the extra cores.
x86 is an architecture basically invented by Intel about 20 years ago. It has been evolving since then. x64, or x86-64, is the x86 architecture with 64 bit extensions. both amd and intel make 64 bit cpu's.