[userbinator]

Definitely DAC or fiber. There is a 25GBASE-T standard, but no actual products exist.

[gpjt]

OP here -- yes indeed. I ever do a new series of posts on upgrading my network to anything faster, the first one will probably be titled something like "25Gb/s Ethernet: how much it costs to rip out your CAT-6A and replace it with fibre".