Using 320.18 right now. Gave me problems when I first installed it on a ~6 month old Windows 8, but after doing a complete reinstall of Windows, it's been working perfect. Must have had so much junk cluttering things, because 6 months is longer than I usually wait to do a clean Windows install. Like to keep things clean. But yeah it's been working perfect now.
I'll definitely update to the next set of drivers, whenever they get released. Always a good thing to keep up to date, unless a particular version is giving you problems and forces you to revert.
xoleras, did you end up getting 2? Staying with SLI, or just one 780?