i7 IS Nehalem:
http://en.wikipedia.org/wiki/Nehalem_(microarchitecture)
The desktop i7s would fall under the desktop bloomfield category. And correct me if I'm wrong, but the server i7s (xeons) cpus don't really offer anything over their desktop counterparts besides ECC, which really isn't a big deal at all. They are literally the same thing besides ecc. There is literally no performance gain.
And no, a mac pro with a nehalem xeon wouldn't be any more robust than an i7 you could build yourself. I have a 920 overclocked to 3.8gz and with my ram at 1600mhz at 7-7-7-20, and it'll outperform any mac pro when it comes down to brute speed. It won't be "far more robust" Quite the contrary, actually.