My Blender scenes are quite large (10+ GB), so because of this, I render with CPU, not GPU (because most graphics cards don't contain that much VRAM). So knowing this, I'm buying a new computer, and am not sure exactly if it matters whether I buy a high end or low end graphics card. I think it won't matter (in terms of render time), because I don't think the GPU is involved at all in the render process (for CPU rendering). Is this correct?
If you are only using CPU rendering then the graphics card will not affect your render times but that doesn't mean that you should buy the cheapest graphics card.
With large scenes you will find the viewport drawing will get slower if you choose a low end graphics card. So while it won't help your rendering you will still want to get a graphics card that is closer to the high end than the low end.
With support for OpenSubDiv the graphics card can be used to speed up drawing objects with a subsurf modifier, this short preview shows the benefits. The graphics card can also used through opencl to speed up compositing.