1.) Will I be able to find drivers for the 6790 in Ubuntu, and how difficult will it be to find software to get them online and mining? (My concern is that setting up the cards/miner will be a repeat of my effort to get my old D-Link wireless card working with Ubuntu ... i.e. 12 hours of frustration ending in defeat)
The drivers are really easy, Ubuntu is very user-friendly and it'll ask you when you boot into it whether you want to install proprietary drivers. If it doesn't, it's a quick search through their software repo.
I've only used Arch (another Linux distribution) so I can't comment on how easy AMD APP SDK is to get, but that's the main other thing you need.
2.) Is it difficult to get both cards mining simultaneously, or is that a standard feature in most mining software packages?
Some (diablominer et al.) run all your cards on one instance of the miner. Others (phoenix) require you to launch an instance for each card. I've written some software (Autominer, in my sig) which you can configure once and it'll fire up all of your miners in a nice user-friendly window whenever you run it.
3.) Does constant mining wear out the cards really quickly? How long can I expect them to last?
It will do if they run too hot, or very high voltage. I think otherwise you're looking at years, graphics cards aren't known for dying early and if they do, you should be able to get them replaced under warranty assuming you didn't flash the BIOS.
