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Realtek's $10 tiny 10GbE network adapter is coming to motherboards later this year
(www.tomshardware.com)
This is a most excellent place for technology news and articles.
Most (all?) 10gbe copper switches will negotiate 1/2.5/5 gbps.
Most 10g switches with sfp+ will as well, but you also have to make sure the sfp+ ethernet module will negotiate lower speeds.
I've had some annoying interactions between 1gbps and 10gbps when using different sfp+ switches and sfp+ ethernet modules. I never dug into it, I just swapped stuff around until it worked.
So no reason not to get a 10g switch to start building things out