For your info, the old Mac Pro (version 2012 and before) can use the 980Ti (internally). IMO, the 2009 model (which has identical spec to the 2012 model after the firmware upgrade) is has the best balance between performance and cost now. It's just around $400, another $150-200 can buy a 3.46 (turbo 3.73)GHz Hex core XEON. So around $600, you can have a pretty fast Hex core (12 thread) Mac that can drive the 980Ti internally. Which is good for both graphic works and gaming.
Any TB equipped Mac may use the 980Ti externally. HOWEVER, there will be a noticeable performance penalty (because the bandwidth is limiting on TB). And eGPU is developing at this moment. It's not something that work OOTB. You can expect lots of issue, not recommended for normal user at this moment.