How many GPU's are you hoping to use in this box?
As many as I can cram in... I count 18 slots, which gives me 36 GPUs
IF I am able to use all the slots, and
IF I can get software to support them. At least one, and maybe even 2 of the slots appear to be "reserved" or may require an expansion board in order to be useable. The other 16 are fair game though. Dual GPU cards would then mean 32 GPUs total.
I have been experimenting with removing the plastic shroud on my 5870s - as you may know, 5870s (and most other graphics cards) fit very close together if you use adjacent slots. There is usually no gap to speak of, which makes airflow impossible unless you use shims to wedge the cards apart. When the plastic is removed, there is tons of space between the cards. See pic:
I have discovered that doing this leaves enough of a gap to fit in an x16 extender, which means that I could mount 8 cards directly to the board, and an additional 8 to 10 cards one level up via x16 extenders, double stacker style. Combined with some special cooling apparatus, this should allow me to air cool all the cards, while still fitting within a 7 or 8 U chassis.
