Is there much difference between 680i/780i and P35/X38 other than the SLI or Crossfire?
Nah, I think reviewers have been summarising the 780i as the 680i+, not really a new chipset, they just boshed the metal in a few places.
Generally, the P35 was streets ahead of the 680i in terms of stability, overclockability, and general problem-free-ness. The 680i was riddled with problems at the start, and its still a mess, with no end of BIOS updates coming out. That said, its not a terrible chipset, its just you'd only really want to get one if you wanted SLI, a 680i would generally do you fine.
I've not heard any bad things about X38 yet, and the 780i is very new, but I'd say that its worth waiting a good 4-6 months on new chipsets and motherboards for these problems and associated fixes to arise. In my view, P35 is a choice you can't fault, unless you are dead set on SLI. And while SLI GT or SLI GTS might the way to go at the moment, I don't see it as a good investment. 2x price for 30% performance increase (at reasonable resolutions), 1GT or 1 GTS512 is more than enough for most things at the moment, and when the new 9 series comes out (not the 9800 botches, but the entirely new G100 core), they will no doubt eclipse GTSLI