The drivers are fine, for every one that has an AMD issue there will be one with Nvidia issues.
To be honest these issues tend towards the top end new cards that are playing AAA titles, SLI and Crossfire figure quite high as well on the list of set ups with driver issues.
A single mid table card should be fine.
I would defiantly go for the 6790 its faster all in all and some times the difference between the two is playable and not depending on the game and resolution.
So on top of what is your power supply I would ask your screen resolution and which games you are looking to play.