Hi,

it's not a problem to use 5 or 6 of these cards in one mobo. Crossfire is not necessary. I run 3 different kinds of 280x i a single rig, but it's quite hard to set all to do the max hashrate. You say you do 650kH/s. On 280x you should do at least 740kH/s. 650 is just waste of electricity;)