Originally Posted by JumboShrimp
I hear it gets pretty crammed should you decide on going SLI on an mATX board...looking for more input before I make my decision.
For a typical mATX mainboard, it has 4 expansion slots. The first slot is usually a PCIex16 slot (for card no. 1). For SLI capable mainboard, either the 3rd slot (eg Asus Maximus III Gene) or the 4th slot (eg Asus P8P67-M) will have to be another PCIex16 slot (for card no. 2).
If it is the 3rd slot, you will have all four slots occupied/blocked as the 2 cards are typically dual slot design. This, of course, does not allow you to install any other card (eg sound card, Lan card...etc). But if you do not have the need, I do not see a problem. It will get hot but should not be critical and could be solved by a good air cooling case with well thought out cooling plan ahead of time.
If it is the 4th slot, then you will have a slot empty in between the 2 cards. Ventilation is much improved if you do not populate that slot with yet another card. But since the graphics card no. 2 will sit in the 4th slot, its double slot width will extend beyond the edge of the motherboard. So, make sure it clears the USB + I/O panel + Power/Reset headers. Also, make sure the computer case has an extra slot opening. This consideration is not unique to a MATX board but to every board with a double slot width card sitting on its last slot.
In general, if you use a mATX board, you would likely to use a smaller case as well. This combination will probably not have temperature as pretty as an ATX board sitting inside a full tower. But you basically exchange this for the benefit of a more compact setup.