My 290x's run at about 90-94C which is the standard operating temperatures. For all earlier cards I wouldn't be afraid of 80's, I don't know where you picked 78C from, I think it can be higher.
Philipma1957 has mined with many cards for a long time, so I would go with his recommendation of 78C as the highest safe temperature given his experience.
Someone (Communism I think) brought up a good point about fans dying faster due to the higher temps of 290/290X on lubricant. I'm not worried about a reference 290/290X because the blower is far away from the GPU and just shoves air down the card, but axial fans located next to the hot fins above the GPU are going to be toasted more than usual. Just because AMD claims 94C is safe doesn't mean it's going to last as long as previous cards. Just look at NVidia's bumpgate--companies may have certain projections of lifespan, but they don't pan out sometimes because of something they forgot to consider.
My Sapphire 7970 fans crapped out on me from heavy usage for less than a year, faster than every other brand I've tried. ASUS and XFX's newest (DCuII and R9 only, not talking about XFX's crappy 7xxx stuff) fans look the most durable to me. Since those fans are semi-sealed against dust, I think they may stand up to high heat better. I have only had good experiences with ASUS RMAs but I've only had to deal with their laptop department and I guess their video card department has a worse reputation.
Sleeve-bearing fans are the cheapest, flimsiest fans and should not be run horizontally, yet video card makers use them anyway, on the theory that it saves the a little money, they are quietest of all fans when new, and by the time the fans wear down too much and get noisy and/or seize up, it'll be time to replace the card anyway. But I think they expect normal use, not 24/7 at high fan speed. I would like there to at least be an OPTION to buy models that use ball-bearing fans, preferably 2-ball bearing, which would last a lot longer even if it's slightly noisier. Look at any good PSU and it will use at LEAST a 1-ball bearing fan due to their horizontal fan mounts; they should do the same for video cards. (Sleeve bearing is fine for vertical mounts like on tower CPU coolers such as the Hyper 212+ since gravity will better distribute the lubricant.)
So among all the 290s that are out so far, I would only go for reference designs and ASUS and XFX, personally. (My 290s are reference.) I have no experience with Gigabyte Windforce but given that they already had a 290 product recall (yeah I know they tried to put the blame on a subcontractor, but still) I'm not sure how much faith I have in their engineering department being on top of things. I'm with Philipma1957/Communism on wanting lower temperatures than what AMD marketing claims is safe.