For AE I would think the most performance u will get at that price is the 590gtx which has 4x the cuda cores and 3GB ram
More cores means faster rendering and then the more ram on the card will help out with larger evironment maps
The q4000 will be much slower
The 590 is basically 2 580s on one board. Depending on your system There's also the gtx480 and 580 where can install multiple cards that AE will use
Engineering is taking my Quadro FX1800 out of my computer. It is on the lower end of CUDA enabled cards but does help. I put up a fight until my boss said I had a $1000.00 limit to get whatever card I want to replace it. I will be upgrading from CS4 Production Premium to Creative Cloud along with upgrading BCC7 AE to 8 and getting the card, hopefully all today.
z600 8core
2.13Ghz
16GBs ram
Windows 7 Pro
Thanks,