Hi Fernando. Ive actually moved on to an HP Z820 from the old Mac Pro.
But in making the move I was faced with a graphics card decision as well.
After studying some benchmarks using various cards with AEs raytrace engine, and comparing costs. I choose to go with the GTX 570. It runs faster then the Q4000 and is at least half the price. Now I dont know about Maya, but Im using it with Blender 2.65 and the software is greatly accelerated. Again, I know nothing about what Smoke needs so I would say do some research there. But all things remaining the same. the 680 seems like a better bang for buck then the Q4000.
Heres some more info that may help you in your decision. Good luck!
https://forums.creativecow.net/thread/2/1021085
https://forums.creativecow.net/thread/2/1019643#1019643
Tom Daigon
PrP / After Effects Editor
http://www.hdshotsandcuts.com
https://www.youtube.com/watch?v=DxPrG3WUyz8
(Best viewed at 1080P and full screen)
HP Z820 Dual 2687
64GB ram
Dulce DQg2 16TB raid