Dual Xeon Questions for a 3D Graphics Desktop

Hi, I built my first desktop about 4ish years ago, and am about ready for a new one. I'm considering 3 main options, but have the most questions about a dual Xeon set-up.

My priorities for this machine are:

1. 3D Modeling (able to handle extremely high-poly meshes, possibly upwards of 20 Million in Mudbox 2009)
2. A hefty amount of Photoshop Work
3. 3D Rendering (less important than modeling because often the renders won't be highly time-sensitive.)
4. After-Effects / Premiere Video Editing / 3D Animation.
5. Energy Efficient (willing to spend a little extra money for this)
6. The Occasional game or movie.


Core components:

---------------------------CPU:----------------------------

2x Intel Xeon L5520 Nehalem Processors (2.26GHz, 60W)
$539.99 each

-What is the difference between the L5520 and E5520? The L lists 60W as opposed to 80W for more $$. Do you lose any performance for that 20W drop? How much cooler does it run as a result?

-Also, how large of a performance gap is there between dual 2.26GHz Nehalem Xeons and a single i7 920 2.66GHz for
1. single-thread tasks? (Photoshop)
2. 2-8 thread tasks? (games)
3. >8 thread tasks? (3D apps & Rendering)

-------------------------Motherboard:-----------------------

ASUS Z8NA-D6C
$259.99

-This board only has one PCIe x16 slot. Would it be able to fit a Radeon HD 4890 1GB? (I have read reviews saying it can be a little cramped.

-Are there any other Dual LGA 1366 boards that can fit in an ATX case?

-This board only has 2 USB ports, is there a way to add more?

--------------------------Memory:-------------------------------

Mushkin 2GB DDR3 1333 ECC Memory x3 (x6 eventually)
$38.99 each

-What exactly is ECC memory and is it worth it? The motherboard is compatible with ECC but I'm not sure how big a difference it makes from non-ECC memory

-----------------------------Video Card:------------------------------

ASUS Radeon HD 4890 (1GB GDDR5 Memory)
$249.99

-torn between this and the HD 4870 x2, or a slower, quieter, cooler, and more energy efficient card.

-Are there any recommended cards that are highly energy efficient and run cool but still have ample performance?

------------------------------Other:---------------------------------

The rest of the components I would figure out after locking down these 4.

My other two options are going with a Mac Pro or building a much cheaper i7 Machine.

The monitor will be the same in any case so i'm leaving that out of the equation

Thanks