Temporary upgrade from GTX580 to GTX780?

Welcome, this is my fist post on this forum, I would like to say helo to everybody. Sorry for mistakes, english is not my main language.

After the last upgrade of Adobre Premiere, my old GTX 580 is not longer supported with CUDA. I have to make some upgrades.

My current rig is:

Intel I7 4770k (stock clock)

16GB (4x4 GB DDR3)

GTX 580 1.5VRAM

240GB SSD for OS

120GB SSD temp

120GB SSD temp2

1TB SSD footage

few HDDs and 24" FHD Monitor.

In the future I have a plan to buy brand new rig (Ryzen 2700x or Intel I9 with 32GB and 1080 or better) but right now I wolud like to spend less money.

Is the GTX 780 3GB VRAM good temporary choise? On second hand market is about 120Euro.

I'm using Panasonic GH5 (150Mbit 8bit), DJI Mavic Air (100Mbit) and have big backlog of FHD footage to edit.

Second question is about RAM. Selling 4x4GB and buing 4x8GB ald DDR3 is not a good idea, right? My mobo have only 4 slots.

Thanks for any solutions. Cheers! Mike