Well, I always used NVIDIA.
Here are some that should be enough (note that they are significant lower than the HD you mentioned, but I believe to pay a bit less for a decent GPU and wait for newer ones to eventually drop price than to pay huge amounts and see their price drop down and think to myself: Should have waited!):
PNY GeForce GTX 570.
Zotack GeForce GTX 570.
Asus GeForce GTX 570. <-- I personally like this one
If you want to go Dual GPU, then GeForce GTX 580, or GeForce GTX 590. If going for 580, then I would go for the quiet Asus GeForce GTX 580.
ON HD series, you could go:
AMD Radeon HD 6970
or
Sapphire Radeon HD 6950
Those are my recommendations, instead of jumping straight to the 6990 series and up. If you indeed want something along the 6990 series of HD, then the equivalent for Nvidia would be the 580 and 590 series.
Do note that Asus is known to have quiet cards, while PNY and Zotack are known to factory over clock the card a bit.
Now for the other components:
Question:
Ain't AMD FX-8150 a 3.6GHz four-module processor, that here has been overclocked to 4.4GHz which is the most common? I know that 2 of my friends got the one overclocked to 4.4GHz and they had no complaints.
RAM: I am not sure if your ram is better than this one: 8GB Corsair (2 x 4GB) Kingston HyperX DDR3 RAM. If it is, then ignore my RAM recommendation.
Cases: Either Cooler Master (got a few to name) or Antec 902 Gaming Case or Corsair 600TM Gaming Case;
Cooler Master cases: HAF 932 Advanced, HAF X or CM 690 II (USB3 or Advanced version). Link to see the Coller Master cases
here.
Cooler (since you did not mention any coolers): Antec Kuhler H2O 920 CPU Cooler or the Cooler Master CPU Cooler.
Hope this helps
