From what I understand:
SLI: either 750i or 780i, 790i if you're rich
Xfire: X48
Single GPU: P35
As for manufacturers, Asus is usually good but commands a premium price. EVGA follows the Nvidia reference design, for better and for worse, except for their 750i mobo. I hear it's quite good and it's non-reference. Dunno any others, personally.