I've been giving this a lot of thought and think I have a better answer.
When we are involved in the creative process regardless of the they, painting, music, video. We want to be fully engaged in that creative process, not constantly distracted by banging into external limits. If we are painting a picture, we don't want to be worrying about how large the canvas is, are the tubes of paint made of lead, will this green fade, we want to be focused on painting, having solved these issues.
When making a video we want it to be the best we can make it, if its a Wedding we don't want to leave a dry eye, a Horror we don't want to leave a dry seat. We want our story, our vision, clearly and fully conveyed. To achieve this we don't want to be constantly constrained by the technology we are using.
Lets look at the workflow, picking it up post shooting. You may have 1 camera or if you're lucky 3. The point is that you should have lots and lots of footage. To store this you need a good high speed scratch and capture Drive. I like that you can swap the internal drives on the Towers, so you can have Drives just for a particular project and when it's done just pull the whole drive. So extendable Drives big enough to hold 1 or more whole projects.
Once we have our footage in, we need to bring it into our editor, most editors use a "Edit Format" so there is on the fly transcoding. This take lots of RAM but more importantly lots of processing. Engines are more important than RAM, at this point.
Now to the Editing, here we are holding lots of large files, RAM is the important factor at this point. If you have lots of large shots all cut together thats going to be RAM. Moving them around, Transitions, Picture In Picture, Colour, thats all processing so we're back to CPUs, pushing lots of pixels.
Once our Masterpiece is done we want to encode it, for Web, for DVD, for Mobile, now the real crunching happens. CPU power is a must, Lots of RAM is good and Lots of Drive Space is a must.
So this is the rub, where are you going to hit the tech barriers?
Storing lots of Footage, having to constantly move big files to "Make Space"?
Crunching the pre-edit Footage, Rendering Transitions, Final Encodings?
Holding large files in RAM, keeping all that Footage available for the CPU to crunch?
The easy answer is buy everything you can afford.
A better answer is try an balance your requirements so you aren't hitting any limits.
This depends on the types of Projects you plan to do.
With a Tower, the HDs can come and go, so you can add capacity as required.
While a lot more expensive you can also increase RAM.
CPUs you're stuck with what you have chosen.
My advice: Buy as many CPUs as you can.
Get twice the RAM the OS asks for, with a commitment to doubling it every 6 - 12 months.
Include the cost of a HDDrives in every project and keep swopping them as your projects change.
