Yes it is possible, but you will need pci extenders and then the max is 8 using a psu of 1200 watt or better.
The other problem if you do not use pci extenders, the max is 6 GPU due to physical limitation on the 890fxa-gd70, and you will need to run the fans at full speed since having the video cards with almost no free space in between them causes the temperature to go up a lot, unless you water cool them, without the pci extenders the last pci slot can not be used since the card does not fit properly on the motherboard since it was not meant for a double width card.
The other problem is you will need to use amd sdk 2.4 and catalyst 11.6 which has a bug that with the mouse that causes everything to lock up once a while if you use the computer while mining.
With catalyst 11.7 it did not work for me. the X and ati utilities will see the 8 cores, but not the miners, strange problem, however catalyst 11.6 seems to work fine other than the mouse bug.
This is using debian squeeze.
So how many gpu's do you run on linux then? That was the real point of the question.