Sorry for the late reply.
Thanks for the info. Why do you think it's not worth $20-$30 to get support for USB Type-C and USB 3.1?
USB 3.1 simply doubles the bandwidth compared to USB 3.0 but few USB-devices need more than USB 3.0 speeds, and most devices get by fine with USB 2.0 speeds.
If you buy a device that uses a Type-C cable, you can just buy an adapter for it and use it in a normal Type A port. And I can't imagine it being important when you only have it in the back panel. Once Type-C is mainstream, you'll want a case front panel port which current cases don't have anyway.
How important do you think it is to get the specific graphic card brands?
The differences lie, for the most part, in cooler quality and warranty.
The best coolers for GTX 970 are MSI Gaming, Asus Strix, Gigabyte G1 (Windforce). EVGA ACX 2.0 is also good, just a little noisier at load. Some of the smaller brands may have competitive coolers, I'm just not that familiar with them.
R9 390 coolers from the three big brands are a bit sub par. Asus Strix apparently has a flaw where only three of the five heat pipes make contact with the GPU. Gigabyte G1 only uses two fans, and can have heat problems based on newegg user reports. MSI Gaming may not handle 250W+ load adequately without excellent case cooling. So, the best picks are Sapphire Nitro (preferably with backplate), then PowerColor PCS+ or XFX DD.
As for warranty, it's 2 or 3 years depending on brand. Zotac (and possibly some others, not sure) offers extended warranty with registration.
Does it make more sense to get an SLI capable board for possible addition of a second matching graphic card later, or just plan up upgrading to a new card when needed?
It's almost always a better idea to upgrade to a new single GPU because of the rate at which GPU tech evolves. If you pay for SLI compatibility as a backup plan, you'll probably never use it.
I see Amazon has the Sapphire card for $345 (with backplate, not sure what that means).
It means the card has a metallic support on the back. Without a back plate, a card with a heavy cooler (Sapphire Nitro definitely qualifies) will 'hang' a bit in a typical setup where the motherboard is vertical. A back plate ensures that the card stays straight (and it may have a slight effect on thermal efficiency). And it looks nice, and oozes more quality
Should 'pascal' factor in to planning next year? I have two 6770 cards sitting here unopened that might be ok until then, or could get a not too expensive card until then.
Pascal factors in insofar as it's fast enough that it's worth upgrading to from a GTX 970 / R9 390. Same with AMD's next gen.
I don't think you'd be happy with two 6770's. Crossfiring two weak GPU's is a bit of a hassle and will result in relatively bad frame times. Plus you'd need a SLI board for optimal results (for PCIe 3.0 x8 bandwidth for both GPUs, instead of x16/x4). 6770 Crossfire is about the same speed as a single R7 265- in a best case scenario where crossfire scaling works well. R7 265 isn't even half the way to R9 390. Also, you'd only have 1GB of VRAM which just doesn't cut it these days.
Sell the 6770's while they're still worth something (unless you can find other uses for them).