Does GTX 770 support SLI?
The GTX 770 shares much of the same technology and features with the GTX 780 including: Enhanced internal water cooling design. Super-quiet operation with new adaptive temperature controller technology. SLI support.
Is GTX 970 SLI compatible?
At 1440p, there’s a definitive lead for the dual GTX 970s – that marks two points for SLI, three points for single-card – at 17.23% > 980 Ti.

Will SLI work on Crossfire motherboard?
First of all you cannot SLI Nvidia cards on a motherboard which supports only crossfire for you to do this you should have a SLI enabled motherboard. Also note that having 1 powerful GPU is much better than having two cards combined.
Does b550 support SLI?
B550s also support overclocking, although you’ll need plenty of luck on that front, as AMD’s chips don’t readily take to such activities even on the higher-spec X570 boards. The newer chipset also supports SLI/Crossfire graphics cards, for what that’s worth, and USB 3.2 Gen 2, which can hit 10Gbps.
How do I know if my motherboard supports SLI?

Look at that, SLI was included in the name. There is often an image of the motherboard AND the box, on the front you can often see a few logos (Intel, AMD etc.) and SLI-ready is almost always included as one of the marks on the box to show that the motherboard supports SLI.