Make sure you get a fast as possible a dual core at least, one core will be fully utilized/needed 100% when folding. I tried with HT on and off, both times one core/thread was fully utilized (per GPU). Didn't really see an improvement with HT off, but due to the limited time I tested, and the highly variable score system of F@H it may not have been a valid test.
Also, for versatility, if you decided to run something else some day, the more cores/threads the better, even with just a single GPU. Milkyway for example, tasks run from a few seconds to a few minutes depending on your GPU and app_config file, so it's common to run 3-10 tasks per GPU, each with a tiny need for CPU. Other projects need quite a bit of CPU to go along with the GPU, but Folding is the only one I know of that 100% needs it's own core/thread.
I love my 980Ti's, but I will once again champion here for the 1070 (assuming you plan to purchase something), with similar or better performance, and 100w less power (TDP), and only a slight bump in price. For me
this was the best deal due to shipping and taxes and brand/model preference.