To get a bit more technical and add a couple more things-
You can only use SLI with certain cards in certain configurations-
2 card SLI-
Nvidia 6600GT's and up (Again, the cards must be indentical)
3 card SLI-
Requires 3 identical cards. 3-way SLI is only available with-
Requires 2 identical video cards with 2 PCB's (Printed Circuit boards) on each.
Nvidia also supports "Hybrid SLI" with some cards. This means using a video card with on-board video.
However, these are no preferable for high end gaming.
Scalable Link Interface - Wikipedia, the free encyclopedia
Here's a Wiki entry for Crossfire. Their are some differences between the two. Corssfire also supports a Hybrid Corssfire system, I've heard great things abou it. I believe you can use a 3850 with it, but I'm not sure what one's you can and cannot. Peep the link to find out more.
ATI CrossFire - Wikipedia, the free encyclopedia