HUGE difference in performance for things like 3D text. Rendering and export are real-time functions, so how much gets buffered will have minimum impact between 2 and 4, but some improvement, minor though. GUI smoothness won't change. Playback may improve very slightly, but not much. That will depend on the power of the GPU, not how much VRAM is has (just like rendering).
One thing that was found out
in a recent thread
is that the maximum frame size is limited by your VRAM in pretty much any application, not just FCPX, so if for some strange reason you are doing enormous videos, (for a billboard? or who knows what.) you might want to take that into consideration.
Bear in mind all the components work together. Tons of VRAM don't mean much if you only have 4GB RAM, or a 1Ghz CPU.
Get the 4GB VRAM, as much RAM (16GB min) you can afford, and the fastest GPU you can afford. If nothing else, it makes your long term investment last longer before you absolutely need more powerful hardware.