ATI/AMD does not release proprietary drivers for BSD, but they do release programing specifications which has allowed open source drivers to be written for Xorg (..but generally Linux sees it first, before it's ported).
I'm not sure how FreeBSD (..which PC-BSD/GhostBSD are simply redistributions) is doing in that area.. but if you avoid purchasing the latest models it should be somewhat tolerable in performance.
Unfortunately, a lot of people do not care about the risks associated with loading proprietary kernel modules.. if you're one of those people you can use NVIDIA devices.
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