Okay, calm down. Let's speak in full, COMPLETE, sentences.
Anyway, I don't know where the x1800xl fares along with the 7800 cards, so let's move on for the moment. SLi and Crossfire are just ways to use two cards simultaneously in order to gain performance. Simple idea. In SLi, you need to use two of the same cores (7800GT and 7800GT, or 6800GT and 6800GT, etc). In Crossfire, I don't know much about it, but I haven't heard too much glorifying it yet.
Anyway, no you shouldn't need any extra cooling.
And will you stop saying 3700+ x2, there is no x2 with the 3700+. It's just the 3700+.
Norm