Not all computers are created equal. There is a significant difference between a desktop computer, which is used for desktop applications like Microsoft Office, and a workstation computer, which is used for design applications like Autodesk Inventor. Design applications are significantly more intense in graphic and non-graphic computations, and therefore require the use of workstation computers. The motherboards, RAM, CPU's, and graphics cards on workstation computers are purpose-built to run these types of applications. If you're going to shop at the big brands websites (Dell, HP, etc...), then make sure you look for the following keyword when browsing: "Workstations" (see images below)
The best approach to finding the best "off-the-shelf" system is to consult the Autodesk website. On the Certified Hardware page, you will see the "Find Recommended Hardware" tool, which will guide you through the process of finding specific computer hardware/graphics cards, that meet or exceed the Autodesk benchmark standards. You have the ability to find System Hardware or Graphics Hardware, based on your selection of a specific Autodesk product or Design Suite.
Simply follow the steps, and you will be presented with a list of recommended/certified hardware for the software that you select. From there, your final task will be to specify the exact system you need from the results given.