Dual Xeons Nehalem ( 8Cores+HT ) Vs Dual Opterons Istanbul ( 12 Cores

Hello there ,
sorry for being a noob in hardware so please bear with me
Iam a 3D artist who plan to open a small studio , our work basicly depends on heavy 3D renderings and Compositing ,
We are looking to custom build 6 workstations ,

Iam confused between
the nehalem [url=http://www.newegg.com/Product/Product.aspx?Item=N82E16819117185][/url]
and the istanbul [url=http://www.newegg.com/Product/Product.aspx?Item=N82E16819105255][/url]
which one is better and faster ..

I only built desktops ... never build servers before ... so iam also wondering if there is rules or constraints about these builds ..
I also read they need specail types of memory ( ECC , Dimm . etc ) which i really dunno about these stuff .. we are looking for 12GB DDR3 ..
As for the Vga we are not looking for quadros , a high end gaming card like GTX 295 will do fine with us , just wondering if gaming cards works with server motherboards .

We need to build 3 workstations for the artists and another 3 as renderslaves ( VGA is not important in that case ) .

I hope someone can help and will be cool if you guys can recommend a full build ( the case only ) and what would be the price of such build if pushased from vendors like Dell , Boxx , HP etc ,, just to know if it worth the saving ..

Best regards ,
M.Rotab