I would see how long until your case comes in stock.
As for the power supply, either will run the system. The RM550 has the advantage of running fanless at lower loads(almost certainly at idle).
If you are satisfied with the performance of a 7790, go for it.
I mean I have a GTX 650 ti from MSI(link
) in my media center system and it is very quiet. It can play many games with decent settings. Clearly anything more demanding requires dropping some settings. The 7790 and that card trade blows depending on the game.
I would just also like to note that watching videos takes less power on Nvidia cards than AMD ones at this time. Another reason the GTX 650 ti is in my media center(system idles under 40 watts and full loads under 180 watts(140-150 for most games) at the wall thanks to careful part selection).
Image from techpowerup