777-300 and -200LR have raked wingtips. 777-200 has the standard wingtip.
5-6% fuel savings is the number I've seen (737-800 and 757-200 w/RR's)
Is the 737-800 wing a newer design then the 777's? I'd guess so. So it's not necessarily just a older wing that would benefit from a winglet.
787 wing flexes about 12' from static at 1G. That, along with modern design and materials, has to help with reducing wingtip drag.