An R9 280 is comparable to a GTX 960, but with one big tradeoff: the pixel fillrate performs much worse than the texel rate. So drawing textures is no problem for the R9 280: go ahead and throw 4k textures at that thing. But pixel effects like post processing, dynamic lighting, and transparency won’t turn out so good.
I’m going to guesstimate that you can get over 1,500,000 polygons on your graphics card and still achieve 60 frames per second. Unfortunately, this rate goes down if you use vertex shaders, and the cost of draw calls for dividing up your polygon budget between so many different objects is quite hefty. Still, even if we halve this rate, that’s approximately 2-3 pixels per triangle. Basically, if your polygons are so small that you can’t even tell they’re there, then you can make a lower LOD mesh to keep things running smooth. You need to worry about your pixel budget for effects at high resolutions more than polygons.