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
Yeah I would like to see those settings too. I heard no one getting over 700kh/s with 280x yet.
I`m currently only running Toxic, but a friend of mine have 3x vapor-x. I belive his settings are 1150/1800/powertune 20/cgminer 3.4.2 and the latest
AMD driver. My toxic runs at 1130/1850= 810kh/s avg. Usually, when going over 1500mem, hashrate goes down. But up again around 1800. Ofcourse you need to finetune around this area, because no gpu`s are identical it seems.