The very basic idea is that two gpu's share the processing effort for graphic intensive data & render one single output, so as to increase graphic performance & reduce load on the rest of the system resources. You can only do it with identical Nvidia gpu's & you need an sli cable to connect the two. IIRC it stands for Nvidia scalable link.
Unless you're playing call of duty four on 60" plasma screens, I would question the need for sli processing but of far more concern is why you have been through four mobo's in under two years.
Putting one foot infront of the other, I would be solving that problem before worrying about graphics - a new motherboard every four months is not normal.