The Administration forum has the best infomation and most informed people (like Anna) on hardware questions.
Check with the task manager/resource monitor on the amount of RAM that you are using with your worst assemblies. If you are using the full 20 GB or close to it, then more memory may help. Otherwise, a faster processor is probably the best way to spend your money, especially if it is regenerations that are taking the time. If it is just a question of spinning a model, then perhaps it is your graphics card.