Does it work well on the AMD 7970 or the 280X? These are the very popular cards a few years ago.
Well, I've only tested it for a short time doing the mining process on an AMD 7970 but haven't quite started full time mining ZCash with it so the results may vary. But since ZCash's mining algorithm is Equihash, it is more based on the amount of RAM that you have rather than depending more on GPU power.
Just in case, you can check out more info about it here: https://z.cash/blog/why-equihash.html
Now, I would also give a shot at Genesis Mining since they will be having contracts to mine ZCash. It is also a good choice if you prefer to skip setting up the mining rig, and giving it maintenance (also paying electricity costs). But, still as of today mining could be seen more as a hobby since it is not very profitable at all.
The Genesis mining contract is too expansive. The 60H costs $2600. With 2x 390x you can get 60H yourself.
The 390x costs about $400 at the moment. That is just 15% of the Genesis mining cost, and you can sell the 390x later.
but yeah, sometimes its "hard" to take that step. but once you learn it, its learnt and you'll be glad you did it