Well, being honest with you, I'm not sure about it myself. I won't get my hardware til then end of this month, so I'm still researching, but I decided within about 20 mins of reading about rigs that I couldn't get away without risers.
I still need to work out the exact ones I need for my board, but the basic premise is "Stick the skinny end into the board, stick the card in the fat end". If you go above 2 cards, I'd recommend powered risers for the others (risers which have a power molex that you can plug the PSU into) or you run the risk of drawing too much from your mobo and murdering it. I can't be any more specific because I haven't worked it out 100% myself yet.
But to boil it down to the basics, if you have 3 or 4 cards, running at 80C each, a millimeter or less apart, they are going to cook each other. The heat simply will not have enough time to get away from the cards before something gets melted.
There are some serious experts on these boards who can help you work out the specifics.
Oh, and wrt to mounting them....people have shown it can be done in a milkcrate, so that's just a small matter of messing with the container (not all people use PC cases
) until the cards are (a) away from the board a bit and (b) away from each other a bit and (c) sitting solidly on something. Alot of people seem to use the box the card came in
Rit