I checked the Antec power supply calculator and it said i'd only be needing about 600 Watts so that's why I picked that
Always be safer than sorry. An underpowered rig can falter and just ruin the 2,000$ you spent on your rig.
Go with at least a 700W. I only say this because this is an i7 rig, so it draws more power. And you'll have room to OC!
And I would go with at least a 700w PSU I think for dual GPU's of that caliber, although the 650tx would do alright...it would just run at a high load, which means high temps, Lower efficiency and shorter lifespan.