Building a system for my nephew, need advice on hardware.

[helusay]

You are way over spec'ed for Rhino. You are fine

http://www.rhino3d.com/4/systemrequirements.htm

Edit: Although for Rhino an OpenGL card is preferred to a Direct3D so you might can get away with a GF GTX unit instead of a FireGL

Yeah, that is what I am reading. That a GTX level Nvidia card will work for it. Maybe for the same price I can SLi two GTX 560s or two GTX 550 (Fermi) cards together.

[f700es]

Is he going to use dual monitors? If not no need for 2 cards is there?

[helusay]

Is he going to use dual monitors? If not no need for 2 cards is there?

Well, when you CrossFire or SLi two cards together, they work in tandem to achieve the same goal on one monitor. It's somewhat similar to having dual CPUs on the same motherboard.

[f700es]

Ah, I see.