you would be extremely hard pressed to build a decent gaming rig for 300 with all the stuff you need.
you could potentially do it, but it would be hella ugly.
and this is an analogy of how dual/quad cores work. its like you and your 3 friends are trying to go somewhere, you won't get there any faster by you four taking 2 cars rather then all 4 in one car. thats how cpus work. one 4ghz cpu will work faster in doing something then a 1.6ghz dual core, that is if the application is programmed for a single core. if its optimized for 2 cores then you can essentially add the two core speeds together to get the speed of it.