I don’t know the specifics for the particular hardware you are looking at getting, but a great source to compare the components such as Motherboards, CPUs, GPUs, Memory and Hardrive Read/Write speeds is:
A few years ago I built my computer selecting the fastest of each component I could find on this site that were of course compatible with each other. The result was great. They have a benchmark program you can download to compare how your system ranks against others on the market.
The main thing for the CPU, is to get one rated as near the fastest for Single Thread Processing. Most CAD software, including SketchUp, do not utilize multi-threaded CPUs, However, many CPU based rendering programs and plugins will, so it depends what your overall software environment looks like.
In my case I am using a strong Nvidia GPU and rendering software that utilizes that (Enscape), so for the CPU, I bought the one with the highest Single Thread speed available at the time.
My system is getting a little dated by today’s state of the art, but still performs well overall.
Also also a tip regarding how much to invest:
In my opinion, if your daily works depends on your ability to finish your projects as fast as possible and you manage to earn a good living doing it, than I say go for the latest state of the art hardware available. The time saved not waiting for your system to respond to tasks will pay for itself.
If you are on a tight budget and can’t go for the latest and greatest, than consider finding or building a system that has components that are at around the top 25% mark. This usually yields the best performance for the amount of money spent and gives you a system that will perform well for a while. The one exception might be the CPU, in that for whatever reason, the CPU with the fastest Single Thread Processor (STP) tends to not cost as much as the crazy mega-threaded processors. So it still might be worth getting the fastest STP CPU available.