When looking for a second, third, or even fourth card to run in SLI there are a few factors you need to keep in mind.

#1 Only the same type of cards can be SLI'd. Meaning you cannot SLI an 8800 with a 9800.

#2 The cards do not have to be identical, although it is highly reccomended. You can SLI a superclocked card, with one that runs at stock speed. However the superclocked card will underclock itself to meet the clocks speeds of the stock card.

#3 When chosing a card make sure the amount, and type of graphical RAM is the same. Again while possible, it is not recomended to use cards with mismatched amounts of ram.

#4 Make sure your motherboard can support the amount of cards, in SLI, you are going to purchase. Not all motherboards can run SLI, and some have limitations as to how many cards in SLI they will handle.