This information from the Redshift site should be just as applicable to FormZ (especially when running FormZ in shaded full mode). The main advantage you get with Kepler/Quadro cards is that more works goes into driver compatibility with all the main DCC packages and in general they provide larger and faster VRAM configurations. There are some really great GTX range nVidia cards available now that provide far better value than Quadro cards.
Redshift requires an NVIDIA GPU with CUDA compute capability 2.0 or higher and 1GB VRAM or more.
An NVIDIA GPU with CUDA compute capability of 3.5 or higher and 3GB VRAM or more is recommended.
Kepler based GeForce GPUs such as the NVIDIA GeForce GTX 670, GTX 680, GTX 770, GTX 780 and GTX TITAN as well as the Kepler based Quadro GPUs such as the K5000 and K6000, work great with Redshift! The Quadro cards are significantly more expensive but are available in configurations with more VRAM than their GTX counterparts. Having more VRAM can improve rendering performance when rendering very large scenes.
The newer Maxwell based GeForce GPUs such as the NVIDIA GeForce GTX750Ti, GTX970, GTX980, GTX980Ti and TITAN X also work great with Redshift!
At the time of writing (July 2015), the best price/performance ratio is offered by the NVIDIA GTX 980Ti 6GB. On the very high end, the TITAN X and Quadro M6000 GPUs provide excellent performance and 12GB VRAM, so they are ideal for users that render very high-poly scenes. On the lower-end, the GTX970 offers good performance at a very affordable price.