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;)
280X Vapor-X can do over 800kh/s with the right settings.

Ah ok...over 800kh/s can you say me how do you get so high hash rates?
I have also Vapor x r9 280x and a single card runs at 730kh/s
In my rigs with 5 vapor x r9 280x i only get 690kh/s
