How good is this build and is it compatable?

Everything is compatible but there is a massive overkill on PSU, even if he had 2 x GTX680 800watts would be enough. For gaming the I5 3570K would be a better choice and the money saved could be used for a min 120-128GB SSD for OS and frequent apps.
 

Rammy

Honorable
i7 is a waste of money for solely gaming, as mentioned above.
550W PSU is fine as long as it's well made
Caviar green HDD as your sole HDD is a bad choice. They are low power usage and performance, better used as a secondary storage drive rather than something you run things from.
2133Mhz memory is all well and good, but in terms of price/performance, it's not very well spent $.
Sound card is probably unnecessary but I guess it's a personal preference.
GTX680 isn't necessarily great value either, but it's still a good card.