I have the 860watt version of this PSU in my computer, it's "probably the best psu ever" according to anandtech:
http://www.anandtech.com/show/5464/seasonic-platinum-series-860wSince I need to have my computer on to mine I will use the computers power supply to power the bASIC's of course, but I would need really long molex power cables (want to have boards around 1.5meter from computer), any ideas where they sell meter long of those or I need to make them myself?
I don't know that (you could really easly do that yourself, just be sure you take great cables).
A sidenote: tension decreases with the distances of the wire. So if you have a 10 meter long wire and you start at 12V, you might arrive at 11V at the other side. I would be sure that i did it right before risking to damage Xk$ of mining device.
finding one will be a miracle, it is intended to be an internal connector. You should be able to get the connectors pretty easily since they have been standard in industrial and computer equipment for 40+ years.
Tension=voltage?
Resistance increases as you go to a smaller wire (higher gauge) as well, so be sure to keep those lines nice and fat since they will be carrying 100W DC.
Tension is another word for voltage. Over a meter the resistance shouldn't be a huge issue. Even with 18 gauge wire you'll only have 0.21V of drop with 10A of current.
If you have a soldering iron or station, just take a cable with both ends, cut it in half, and solder a length of 14 gauge between them.