Originally Posted by tictoc
The bottom slot on your board is a x8 slot, so there is nothing wrong with your setup. The x16 slots in your board are the 1st and 3rd slots.
To get the absolute maximum performance out of your cards you would have to move the 280x into the 3rd slot. If you are on air this will probably make your 7970 run very hot, since the 7970 will be intaking air right off the top of the 280x.
Running your second card in the x8 slot will not be noticeably slower than running it in the x16 slot. You may run 1-5 fps slower in the x8 slot. With the 280x in the bottom slot the 7970 will run much cooler (and OC further), so there really would be no difference with the 280x in the x8 slot.
Thanks for the help me and my brother are had an argument , he said the 7970 280x cf does not work I did so some tests and benchmarks I scored 21000 in 3dmark 11 but the reason I got the 280x is for bf4 I run it on ultra my avg fps is 70-80 highest around 150 (open maps ) my loweat fps is 47 I don't know if it's my setup or drivers/game that causes such spikes