Update on the graphics card I purchased recently:
I turned the settings down a tad, turning off a couple of the more intensive features such as Bloom and Shadows but leaving textures and shader quality at High. It still looks gorgeous but how does it handle lots of objects on the grid?
Well, Friday night's mission showed virtually no client side lag and my FPS remained high the whole time even when there were 30 rats and multiple objects. And running two clients windowed also showed no lag and switching between them was practically seamless.
I'm more than pleased, I'm ecstatic! This was well worth the investment.