Hi!
I think I will get a new graphics card for Christmas so I wonder, what graphics card is best for rendering? Is it just like “get the best graphics card you can afford” or can a GeForce GTX 580 be better at rendering than a GeForce GTX 680 for example even though the 680 does better in games etc.? What is the best graphics card for rendering that I could get if I had a budget on like 600$?
Thanks! 
For $600 you can look at GTX770 4Gb or even better - GTX 780. Either one will be fine, though GTX780 will be considerably faster as its almost a Titan (never buy old hardware unless its one generation before, at this point there is no reason to look at 5xx series cards at all).
EDIT: That system agnostic faq is a bit out of date - it does not take into account the new 700 series cards.
Okey, so which one of these two:
http://www.prisjakt.nu/produkt.php?p=1952161 (Gigabyte GeForce GTX 770 4GB)
http://www.prisjakt.nu/produkt.php?p=1936005 (Gigabyte GeForce GTX 780 3GB)
should I get? This 780 got 1 GB less memory and is like 150 dollars more expensive than the 770. Is the memory very important for rendering? I’ve had some problems with it when I was rendering a scene I made, it said “Cuda out of memory error” or something like that, and to solve it I needed to reduce the subsurf level on stuff in the scene. Is the memory very important for rendering? Has the 780 got that much better performance that the 1 GB less memory wouldn’t hurt?
So which one do you think would be best out of these two?
The 770 is essentially an overclocked and rebranded GTX 680, which was a bit of a disappointment CUDA performance wise. So yes, the 780 should be significantly faster. But what good is speed if your scenes don’t fit into memory?
Unfortunately nobody but yourself can answer which feature to favour (speed or memory), since we don’t know what you want to achieve (complexity of your scenes / textures / etc.)… Why don’t you render out some of your most demanding scenes and observe the memory footprint they produce. If they stay beneath 3 GB, you’re good to go with the 780. If not, well, there’s the 770 - and there’s still the Titan…
There are also some undying rumors about a mysterious 6 GB GTX 780 Ti about to be released. Not sure what to think of it, but maybe it’s wiser if you just got a gift coupon for a new graphics card for christmas and wait a few more weeks until the marketing fog around the 780 has lifted a bit.
There sadly won’t be a 6GB version:
I read somewhere that there`s NDA on 6Gb Ti version, might be still possible sometime later.