I have 2 120mm fans blowing on my 2 8800 GTS 512.And they work great.the cards are OC and the card fan is set at 40% thats inaudible.They NEVER go over 66 degres.And also there is only 2mm space between the cards.
With a decent case it shouldn't be too hard. Probelm could be the stock VGA coolers for some models.
For me, I run sli 8800GT and an OC'ed Q6600 in a small quiet Sonata II case, so it took a little work to keep it cool in hot Summer months.
1) Zalman VF-900's instead of the single slot 8800GT coolers.
2) Modified the Sonata II intake ductwork to feed the graphics cards instead of the CPU.
3) Installed an AC freezer 7 pro to cool the CPU instead of Intel cooler (that needed that air intake blowing into it.
That's it really. Load temps for the 8800GT's are now both 60-65 vs 80-90, and I'm using the included fanmates to lower the voltage/rpm to the VF-900 fans (so gpu temps could still be lower running 12v).
Thermaltake Armour case is working well for me with the optional 25 cm fan on the case window/door.
I also added the Thermaltake Cyclo slot fan which I have installed between my two 9800GX2's to exhaust the hot air between them more efficiently.
Bumping up the fan speed on the cards works well too.