sli is bad because it doesn't use the 2 cards to its potential. It's better to spend the extra money you would spend on the 2 cards and buy one better one. and if you want to run games at max settings on big monitors it would behoove you to get a sli set up, or even better crossfire (ati's version of sli) but ati cards are expensive at the moment.
SO! for all that talk, don't get sli unless you want to brag or play games on max settings at high resolution.
some things you'll want to look for in a mobo are:
-SATA/IDE slots, whatever you plan on using for your hdd. sata is faster
-if you have money to spare, you might look into getting either a sli board, or a board that has 2 pcie slots for crossfire.
-make sure the fsb on the mobo isn't faster then the fsb for the cpu
-make sure the mobo and cpu are compatible.