This time around the Sapphire Toxic is the best R9 280X, at least in terms of gaming performance. It's not the quietest or coolest card but it's the most powerful. It's also much cheaper than most R9 280X variants, at least in the UK, so it's win-win. The guy above recommends the Asus Direct CU II but while that is a great card, it's more expensive than the Sapphire Toxic and it's less powerful.
Your PC will deal with it fine, and between those two PSUs you should get the 600W version if you're getting a CX. You'll want a 600W PSU regardless of which version you get - the card does need the power.