I would leave the processor & motherboard, both things you have selected are fine.
You need a decent cpu cooler, the standard cooler which comes with the cpu will just not cut it when gaming so processor will not run at its best turbo clock.
http://www.overclockers.co.uk/showproduct.php?prodid=HS-002-EA
I would also look into a good brand thermal paste (arctic cooling) as that is the crucial connection between the heatsink & cpu.
A decent heatsink is crucial when gaming because the cpu will get hot, and once the system detects it's running at that temperature it would drop the ghz to keep it cool in term reducing performance.
As mentioned a decent ssd is crucial, not just to load OS but to save your games on it. You will find the big 20gb+ games will run much more smooth on a ssd than a hdd. The difference in loading times is also insane.
Also mentioned is the psu, if you want to stay future ready buying a 700-850 will keep you well prepared for running a bigger graphics card or running sli, the current 550 you selected will not be able to run sli or a bigger card than you have selected.
If you can stretch the budget a bit on the graphics card as that's the heart of your gaming, if not they are also sli or crossfire ready so in future you can just pop another one in & bridge it.
Another crucial thing for a good system is ventilation, you want to buy 2 120mm fans for the front of the case which suck cool air in & at the back of the case you want 2 120mm fans which sucks the air out. This push pull setup provides consistent cool air in the case for the cpu & graphics card.