Yeah, the 280x seems a good fit for what you're looking for.
You should overclock your processor. It's a great boost in multiplayer games, and the only thing you need to pay for is a heatsink.
For AMD vs. NVIDIA, it comes down to whether or not you will use the NVIDIA features like Shadowplay and G-Sync. If not, then the 290 is a better buy than the 780. But they're both great cards.
Your PSU is plenty sufficient.
Check out the Phanteks Enthoo Pro from the OP. Check out the other cases listed there too. Those are all the best.
Faster RAM *might* make a difference in the same types of games where overclocking matters, which is more or less multiplayer games, Source engine, Blizzard games, UE3, strategy games, and some others.
Yes, you can reuse your windows key for the new build.