It is a bit more complicated:
SketchUp uses one core (like all 3D-modellers). When you alter something in the model, it needs to check what influence this has on the model. This can only be done in a serial sequence, so it needs to ‘iterate’ the whole model with each edit. (Choose a processor with a high Ghz)
Depending on the model size, it would use the equivalent amount of RAM. If you use extensions to model, that amount could easily be multiplied by a factor of 10, for instance.
When orbiting in SketchUp, the model needs to be ‘pixelized’ for the screen. Then, the GPU is used. Hooking up HD monitors will have an impact on that, the total number of pixels of the hardware is leading, Graphical memory of the GPU is binding.(in general, the higher, the better)
V-Ray is a ray-tracer. Those are basically simple calculations of vector-projections, it takes a pixel in the screen memory, set it to a color and then iterates the model to look if anything could change that original color by calculating all the rays that reflect, retract etc. The more iterations, the higher the quality.
In V-Ray, you can choose to use the CPU for these calculations, or the better equipt ‘CUDA’ cores that come with Nvidia architecture. Note that only Nvidia graphic cards use the CUDA technology.
Check the benchmarks for V-Ray: