I definitely agree about the RAM. My new laptop seemed to make a big jump in performance when I added the second gig (of course it wasn't long before the upgrade so I this not based on extensive experience or careful testing). This seemed especially true in Windows.
I do think that multi-processor and/or multi-core is a big deal. Even though AC doesn't use more than one (except for Lightworks rendering - which is not insignificant), it does run significantly better when it has one all to itself. It becomes even more important when running multiple applications. Whether that is multiple sessions of ArchiCAD, or running along side Word, Excel, Photoshop, etc...
Whenever I have used multi processor systems I have noticed a significant boost in user productivity.
If you are only getting one machine you might consider a laptop. I have been buying only laptops since 1998 and have had no difficulty getting the work done. On larger projects I have often had the use of my client's workstations and I have noticed that, while the extra power is nice, productivity does not increase in proportion to CPU speed. For single residences (even big detailed ones) a modern laptop will work just fine. I worked on an 18,000 sf house with full traditional ornament (modillions, dentils, crowns, friezes, entablatures, paneling, etc. inside and out - not to mention the doors, windows, casings, baseboards, casework etc) on a computer with considerably less power than my current (new) laptop.
The added flexibility of a laptop can be a huge benefit. Besides live design sessions with clients (a portable projector really knocks them out) it allows me to work at times and places that I otherwise could not. This contributes considerably to my quality of life. And you can still tie yourself down to the desk when you really need to crank out some work on a nice big 23" to 30" display.
I haven't had the chance to test it yet (I will soon), but it seems that my CoreDuo machine is quite comparable in speed (real world usability not necessarily CPU benchmarks) to my clients' brand new HP workstations.
If you are really feeling adventurous you might even consider the new 17" MacBook Pro (I am VERY happy with mine). It costs a little more as a Windows machine since you have to buy the full retail version of WinXP. It is otherwise pretty much the same price as a comparably equipped Dell and is so much nicer to look at (and carry around).