Robert's request was to build this computer with a budget of $900 that's tailored for Autocad 2012, Revit, and 3D Studio Max. I can't in good conscience make this budget, but I can get close. With some price matching on the end builder, you can probably get it in budget. 3D Studio is the biggest resource pig. I'm recommending a build with 16GB of RAM, a 64bit multiple core CPU and a GPU that meets the specifications according to the manufacturer.
If you would like to request your own custom computer kit, fill out the build your own computer kit form.TIP! A Windows installation DVD, a monitor, keyboard, mouse and speakers are not included with this build.