Careful, he's not talking about a 7790, but a 7990... totally different thing! 375W TDP.
Are you sure you need that much GPU horsepower? It's overkill for gaming at 1920x1080 @ 60Hz. If you have a 120/144Hz monitor, or high res, it might be worth it.
That 7990 is actually a dual GPU card, which means it's actually two graphics cards on the same PCB. You rely on drivers to get the performance benefits and it's never actually double the performance of a single card. You might be better with an r9 290, which is a little slower than the 7990 when it's performing optimally, but you won't have the driver issues you will with a dual GPU card.
One more thought, you have a mATX motherboard? How good is the airflow in your case? That 7990 dumps loads of heat into your case and you might find it turning into an oven if your case isn't up for it.
Finally, you'd want a good quality 650W PSU IMHO as a minimum for a 7990.