Reading up a bit Quicksilver uses DX. What this means is it will be highly driver dependent. As Imouto suggested you would be best to check with other Max users to see what they recommend. While it looks like either brand hardware wise will do, you need to find out which vendor (AMD or nVidia) offers the best driver support, or the app is better optimized for, and users are the best to check with. If both work equally well, and you prefer AMD then a 7970/280X should be a good "sweet spot" option as Tahiti is actually quite a well rounded design for both graphics and compute.