The 650W Corsair would be just fine with two 4870s and your 940. Even with an overclocked i7 the total power draw of a system with two 4890s in CF is less than 500W. Still, the PSU will generally have a longer life if you don't run it too close to it's max so you may be better off with the Corsair TX750W. If you check out the game AMD website you will see that it's certified to run two 4890s.