Windows= 8 GPU (Dual Gpu cards count as 2)
Linux= 10 GPU (I personally have not tested this because I do not run Linux)

Last August or so ATI updated the drivers to allow more than 4 GPU's (or 5...cannot remember).  Windows would recognize them but ATI's drivers would not allow them to be used.


Also, I have been using PCIe extenders with no issues for months...5 GPU/Machine with 4 extenders.