i am going to replace my macpro with a new macpro can the amd cards be replaced with nvidea cards? i use octane for 3d rendering and need all the cores i can

Firstly maybe you should get a 2009 instead, upgrade to 3.4GHz 6-core (or do you need 12-core?) and get GTX 980 instead, the 6,1 cannot swap out AMD Dxx (assume you want the BTO $1000 D700 too).

That and BTO $1000 1TB SSD on top of 6- or 8-core can buy a lot of good solid Classic Mac Pro, and one or two GTX cards.

Reasons why Mac may not be good candidate: even using two GPUs means extra power PSU.

For awhile I was hoping to see Thunderbolt2 expansion box to support 3 x GTX 980s (as an example)

Hard to imagine 8-cores isn't good enough though newer Xeons E3 or E5 do more clock for clock cycle, those HT logical cores are only addng 25% (5th core to a 4-core system). And while the 2008 is old, 1600MHz fsb, DDR2 800MHz, it is GPU and PCIe 2.0 that is its weakest link probably.

I found this from Amandtech:

least should be much faster and much cheaper than many many-core CPUs. 4 GTX

670s would be about $1400, and could fit in a standard LGA1155 machine (with

high-end mobo). - does Octane work for everything the CPU can

do?

and on another site (more for my own education):

"... Nvidia has recently announced a GPU capable of supporting 24GB VRAM [the GK110], which is quite respectable, and we do see a lot of promise. There still remain significant challenges such as the lack of standardisation around CUDA, OpenCL and so on, or that code needs frequent updating for specific hardware revisions, but we are evaluating GPU rendering closely over different parts of the render pipeline."

"Good examples of GPU rendering solutions include Chaos V-Ray RT, Otoy Octane Render, cebas finalRender, Art & Animation Furry Ball and our own Nvidia iray. There is also an emerging GPU renderer called Redshift [which supports Maya and Softimage] that looks very interesting."

http://www.creativebloq.com/3d/whats-future-3d-rendering-21410582