Ample video card memory is good for textures sets, if you look at the high end nvidia (and ATI) stuff you're looking at 1.5gb of memory minimum, more for the expensive cards.

As GecT noted, you also need to see how quickly the card can shift all this texture & poly information between the video processing unit and the memory on the card and the PCI bus. This is the transfer rate and the higher the bandwidth the more information you can shift. (the new Quadro 4000 can shift about 90 Gigabytes of info per second!! the 6000 can do 144GB/s! )Try for 256 bit or 384 bit as a minimum memory interface bandwidth.

For rendering, more cores (as fast as possible) + lots RAM is your friend. The core i7's are good at this, and has a very low latency memory interface! Data in quickly -> Data out quickly!