Depends on what types of games you play, for stuff like GTA4 a quad core is necessary, RTS games tend to appreciate a quad core, but most FPS games are alright with a quick dual core. There is too much variation between games to make a generalization about what CPU is necessary, but for the most part a quick dual core is sufficient but more CPU power isnt a bad thing.
GPU is dependent on your resolution, motherboard choice is determined by your CPU choice and if you want to run multiple GPUs in the future, HDD has no effect on in game performance just load times, and 4GB of ram is sufficient, higher speed and lower latency is good but wont make an extremely large difference where as putting extra money into the GPU rather than the RAM makes more of a difference.