i want to build one 42GPUs super cracking mechine

[epixoip]

cluster of this size will cost you about $60k to build and draw about 13kW of electricity. if you're still serious about building this "super cracking mechine," then check out:

http://www.youtube.com/watch?v=3axK5P8xw-E
https://hashcat.net/wiki/doku.php?id=vcl_cluster_howto
https://hashcat.net/wiki/doku.php?id=linux_server_howto