A hashcat user build himself a crazy box with 7, i repeat 7, nVidia GeForce GTX 580 in it. The box pulls about 3000w and is water cooled.
To the performance:
oclhashcat+ multihash NTLM rulebased 11,3 B/s
oclhashcat multihash MD5 hybrid 9,8 B/s
(both against 500k hashes)
oclhashcat takes about 7,5 days for the full space of -1 ?l?d?s?u ?1?1?1?1 ?1?1?1?1
Here is a pic of this sick box: