Which video card to use with my GTX 580?

Hi everybody. I could'nt find an answer to my question in the forum.
I'm a 3D artist and Nuke's compositor. I'm starting to play with Resolve at work and I had already find this software really amazing. However, I know it could be a lot faster if I had the good configuration. For the moment, the PC I work has:
Motherboard - Asus p8 Z68 - V/Gen3
Processor - Intel i7-2600K 3.4GHZ
Ram - 16GB
Graphics Card - Nvidia GeForce GTX 580

So, to begin, I would like to buy a second card. Probably a better one. So I could use the gtx 580 as a gui card and the new one for image proccessing. Is this correct? My first question: Wich card would be perfect for my configuration? a not too expensive one please...

Second question: When I try to do some renders, my videos got some black frames, and in the up side of the image, they got a strange flicking? this may have somtehing to do with the PC configuration, lack of a second card, or memory, maybe?

Third question : It's really impossible to export Quicktime PhotoJpeg from davinci? They don't plan to include this codec in the next versions?

And last one : I got this message failed to initialize digital powerhouse with the configured number of decoders. What this means?

Thank for your attention guys, and really sorry for my english!

Get another 580, But if you have money to spend then 690. That is the only card that can beat 2-580s

thank you for your answer, I'll keep that in mind!

For a GUI card that also has to work in Nuke, Maya and Adobe apps, the Quadro cards are great! Like a Quardo 2000 or 4000.
Then when you run Resolve, the Quadro runs the GUI and the GTX you have runs the processing. GTX cards work better for Resolve and Quadros work better in 3D apps and Adobe apps.

Hope this helps

Hi Jon, really apreciate your help! The Quadro + my 580 is a good option. I'll think about it. I'm waiting for davinci 10, the config guides should come out soon. someone told me the gtx titan would be an excellent choice as single card solution. I'll wait to see!!!

If your looking to really up your performance the GTX-690 or new Titan are the way to go. The only thing about these cards with a GTX-580 is that you will need a big power supply. I would reccomend 1000 Watts minimum.

Hope this helps.