Should be able to run 2133 and that will be about the top end unless you load extra voltage to the MC (memory controller) and/or OC a bit through the BCLK, but I wouldn't really spend more for anything faster
Intel states that the Processor's IMC can handle DDR4 ram at 1866MHz so no they will not run at 2666Mhz but they will downclock to 1866Mhz regardless of what kit you purchase. In hindsight I'd like to ask you what motherboard you're looking to pair the Pentium G4500+DDR4 ram?
Should be able to run 2133 and that will be about the top end unless you load extra voltage to the MC (memory controller) and/or OC a bit through the BCLK, but I wouldn't really spend more for anything faster