That's what i was missing. thx.

I've seen dedicated mining machines with 4 video cards installed, but I can't seem to find cheap mobo with more than 2 PCI-E slots. Do you have to purchase a high end motherboard to run over 2 cards on the same pc or am i missing something?

Don't know about nVidia but for ATi, the o/cing issue is anything but clear cut. I mean, CCC allows you to o/c your cards (within certain limits, but how can one know if the limit was breached?), and i doubt manufacturers would come up with warranty conditions that contradict ATi's official support.

use AMD GPU Clock Tool, you can directly input ram and gpu speed with it
i haven't seen an update in a while but the latest version supports Cypress gpus