Hardware requirements for AutoCAD Architecture 2008 (ACA 2008) and many other software's are quite high. For example RAM 3 GB (Recommended); 2 GB (Minimum). To work with 3D in Vista using AutoCAD 2008 also requires at least 2 GB.
Is it a good idea then to get 4 GB? Maybe not. Take a look at Coding Horror: Dude, Where's My 4 Gigabytes of RAM?