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.