actually yes, SLI & Crossfire are for all intents & purposes the same type of technology, but you can't run 2 nvidia Cards in SLI in a Crossfire Motherboard & vice versa.
Geforce 7950GX2, & 9800GX2 cards require an SLI compatible Motherboard.
a while back, a year or two or so ago, at the end of the Radeon X1950 series' life & beginning of the HD2900 series, ATI... well not ATI, but one of the Manufacturers of ATI cards, Sapphire I believe, came out with a Dual X1900/1950 based Card, that required an ATI Crossfire board to use both GPU's, otherwise it worked like a single GPU card... wasn't officially supported by ATI either, so I'm not sure if drivers ever came out from Sapphire to support Quad Crossfire for it or not.
however, I'm not sure if ATI fixed that with the 3870x2, whether it requires a Crossfire compatible motherboard to use both GPU's or not? I know the 4870x2, doesn't require it, but does if your going to use 2 of those cards... so you could run a single 4870x2 on a non-Crossfire system.
but can't seem to find any info on the 3870x2 in regards to the same thing or not?
in any case, you should still be able to get some sort of display, whether one or both GPU's are being used...
could it be the monitor cable is the culprit, do you have another system to check the monitor with to make sure its working?
as to the fans, if their set to automatically speed up & slow down (throttle) based on the GPU temps, that could be the cause of that speeding up slowing down bit, but if their physically stuttering, as in stopping & going, something is wrong there.