Well, I can say it's unlikely you're going to find very many
who use or even know what Z-Brush and / or Maya is

The 8800 card. Stay away from it. It has known OpenGL issues
atm and it will absolutely kill your camera response within
Z and Maya both. I shipped mine back. I'll put it this way,
my LAPTOP with it's slow as hell 5600Go chip in it crushed that
board when it came to rotating a working shaded mesh in XSI. The
issues were THAT bad. They may have resolved them by now, but
check the Maya / Z forums to make sure before you commit.

I wanted to game on my system so I didn't go with a Quaddro board.
You will not game with a Quaddro. Just know that right now

Had a Pro board once. Cost me $1600.00 at the time. Back when
the Pentium Pro 200 was bleeding edge, I had Windows NT box
running dual PP-200's, the Oxygen 404 gfx board and Lightwave.
About $8000.00 in hardware, I ended up giving it away years
later. . . . .

My personal opinion ? Don't go Quaddro unless you work in a
studio with Maya and Z. Then let THEM buy you a quaddro if they
feel it's necessary. I run XSI, Z, and Rhino on a 7950 GT OC
card and I can't be happier with the results.

Don't go crazy on the Ram just yet. Unless you're running out
in Maya or Z I would pick up just four GB initially. Run the
apps for a while then determine if what your doing actually
requires more or not. I run mine with 2GB just fine

A 600 watt power supply should do you fine. Make sure you match
the power supply to what your graphics card needs. ( when you
decide on that ) Some cards require dual 12v rails that meet or
exceed certain amp requirements. Some cards require dual power
connections and some of those may be four or six pin depending
on the card. ( Mine is a single six pin, but the 8800 required
dual six pins )

I have zero experience with the quad cores. I run my system on
an AMD FX-60 dual core myself. Not sure of the heat they output
nor how much power they consume.

Hard-drive: I would probably get twin 250's or at the very least
a secondary drive to backup your data. It's easy to re-install
the application, not so easy to rebuild that model that took a
billion hours to get right. . . .

XP-64. I would be running it, but I think I mentioned I like to
game on mine still Make sure it has drivers for your gfx board
and other goodies you'll pick up.

Try not to overclock it ( as you're looking for stability here
not bleeding edge speed ) and don't forget to pick up some cooling
gear for all that heat is going to have to go somewhere

-Daye