We ended up in choosing the 5970 against the latest firestream for its better performance/money ratio.
I would like to do that too, but unfortunately it won't work. As of Stream 2.2 the second GPU on the 5970 is not supported. The problem seems to be that it is locked into crossfire mode and cannot be used for compute.
I would like to build a workstation for scientific purposes with 4 card HD 5970 in crossire configuration under Fedora core 12 64bit
Crossfire prevents use of more than the first GPU for compute; you always want to disable Crossfire for compute applications. Crossfire configurations of more than four GPUs are not supported anyway. Unfortunately as noted above it seems impossible to disable Crossfire on the 5970.
is there any architecture constrain? (e.g. max addressable GDDR5 ram, intel vs AMD mobo chipset)
The main problem is BIOS support. For example the BIOS on my EVGA SR-2 motherboard is only certified to support 7 GPUs.
I have the feeling that it is going that the case is going to be very hot, any suggestion?
Use water cooling.
Fast estimation tells me a need at least 1000W power supply, any suggestion about it?
The Radeon 5970 has a max power consumption of 300W, that's 1200W for four plus the power for your motherboard and CPU. I would use two 800W power supplies with a dual PSU adapter cable... but as I said, this setup will not work at all unless and until AMD fix the 5970 useless second GPU bug.
CFX disabling on X2 cards used to require Catalyst AI OFF
Don't know if it remained that way.