LG 27" On Mac Pro 2013 Quad Core Question

I am looking to buy one of the new LG 27" 5K monitor they sell at the Apple store and was wondering if the D300 2GIG video cards that came with my Mac Pro will support the 5K display? I have read in some forums that the current video card I have will only support 4K and others say that apple did offer 5K support with this cards?

I have also read the the Dell UP2715K will work if you use the dual thunderbolt cable setup? I would like to go with the LG because it has the camera and is setup to integrate with the mac software like the older mac displays.

I currently have a Mac 27" 2K thunderbolt display installed in my system which is a beautiful display, but looking at the 5K option. Any suggestions would be appreciated. I looked on the mac forums on the LG 27" connections and it looks like it does not support dual cable connection, so I am thinking I won't be able to get 5K video with the dual D300 video cards I currently have?

Ron