Upgrading graphics card

I need help upgrading my graphics...my current spec suffers from lag when editing 4k footage...im guessing its down to the gpu?

128gb solid state hd
32gb ram ddr3 1333hz
Nvdia geforce gtx680 2gb
2.8ghz 6core intel xeon

Aside from the Mac Pro tube, not much will let you upgrade, outside of an eGPU which works only in specific situations.

When it gets problematic for true time playback and timing I'd create proxies. It's silly to struggle with stutters and stalls playing 4K footage when initially your interest is in getting the pacing correct. Use proxies until you get the thing nailed and render out using original 4K media. I establish the grade off the 4k but don't bother playing the 4k since everything's been established and locked tight editing the low res proxies. 

What machine is it? Old 4,1/5,1 machine?
What macos are you using? High Sierra because of Nvidia graphics?
Not certain but something like this: Radeon 56/64 for High Sierra, Radeon 5xx+vega VII for Mojave, Radeon 5xxx for Catalina and above.

Use proxy video?