Well I'll get 7 280x at 117$ per pieces.. 4 are gigabyte version and the rest Asus.... They where used for mining and some of them still have warranty.. It's that a good deal?
You will have a problem finding a large enough power supply for 7 280X cards. While it is possible to use dual power supplies, they may not provide a stable environment. If you are in the USA, plugging the rig into the socket(s) for a single room will load the circuit to near capacity. 280X cards do a good job hashing, perhaps ~310 Sol/s.
R9 Fury cards are more expensive, but more power efficient and hash at ~460 Sols/s. A single 1600W power supply can power 4 R9 Furies and 1 or 2 smaller R9 series cards. This is for mining Equihash, algo, not "Dual-Mining". Dual-mining may burn up the power supply with 4 R9 Furies or 6 R9 280X cards. You will run out of connectors for 6 or 7 R9 280X cards and a single power supply, also. A power supply should not be loaded to full capacity if you expect to mine without it burning up.
Larger rigs are often less stable. If this is your first build, start with 4 cards and expand. Larger rigs are impressive, but start simple first. Two stable four-card rigs with smaller power supplies will produce more hash at less expense, and allow you to turn on a light or make toast. Place the rigs in separate rooms. If you are in a college dorm, that will save the embarrassment of constant circuit overloads. --scryptr
