Hi, this is my first time build a gaming PC. Am I missing anything? Are all of these parts compatible with one another? Will the case be the right size? And how well will this computer be able to perform on games? (Crysis 2, Starcraft 2 Ultra settings, Battlefield 3)
The 6870 isn't the most powerful GPU. It will run the games you listed, but it certainly wont max them out with a good FPS. You need to be looking at more like the 6970/GTX 580 for that kind of performance.
With that ASRock MATX board you cant add another card, so an 850w PSU is just insane overkill. 500w is more than enough.
If your buying the i5-2500, NOT the i5-2500k save yourself some money and go with a H67 motherboard and just use the stock cooler. Otherwise stick with a z68, slam a 2500k and over-clock the beans out of it