Ehh.. SLi boards... they are quite buggy right now...
I prefer Crossfire boards because you can use really nice P35/ P45/ X38/ X48 boards which will get you better overclocks, also the fact is that ATi's cards have really good bang for the buck thanks to the release of the HD 4850/ 4870 but I thought you wanted to get Crossfire because you mentioned HD 4870/50
750i FTW, 780i's are good boards (by those bugs I mean the video corruption bug, if you Google you'll get tons of pages giving a thorough explanation on it) But in fact nobody here has recommended either of those boards in a 2 or 3 weeks haha..