AC8.1 will run good on both high end ATI and Nvidia cards.
I'm running AC8.1 on a rig with a ATI FireGLZ1 and the other with a Nvidia FX5700. Both run smooth in 2D and 3D.
I belive that as long as you have a mid range video card and lots of system memory (1-2GB min) AC8.1 will run just fine.
But I 'hm also a proponent of dual cpu machines. That makes a world of difference in todays multi-tasking workstation.
and with all the newest generation video cards out now.......
You can pick up the FX5900s or 9800s pretty cheap now. I'm one who doesn't think I have to have the latest-greatest-most expensive computer parts to do my job. Last generation parts are A-Ok by me.