The role of the graphics card in SketchUp is often overrated. It can very well do with a cheap card, provided its driver has the required OpenGL support.
Roughly speaking, the graphics card is used for raster-based functions like textures and shadows, while the number crunching required by the geometry (edges and faces) is performed by the CPU. As SketchUp is a single-threaded application (it uses only one of the processor cores) the real bottlenecks lie with the CPU. An ultra-fast CPU (high clock speed etc.) with a mediocre graphics card gives better performance than a lacklustre processor with an ultracool whizbang graphics card.
The idea with the “professional” cards is that their drivers are sometimes better than those of the “consumer” models, but usually a “gaming” card has more bang for the buck.
Anssi