UPDATE on the assessment of the Garibaldis - apparently Jane's and Renao Sicurezza agree that the belt was Harvey (making it equivalent to 12" of wrought iron). This means that the British 12" 35-ton RML gun could penetrate it at battle range, and that some British 6" guns could penetrate it at battle range too (2,000 yards) though the 6" QF gun was not among them.
So no - worthwhile ship, harder to sink than a PrC of the same rough size and armament, but not a revolution. I suspect part of the reason they were so effective when used by Japan is that the Japanese gun crews were so good and that the Russian fleet was rubbish. (e.g. so overloaded that the belt was underwater.)
And while I'm at it:
Very much so. The first Marceau was ordered in 7 Oct 1880, laid down two years later and commissioned in 1892. The Marceau was ordered well before the Admiral class and commissioned barely ahead of the Royal Sovereigns.
I've done some reading about the Russo/Japanese War, the really only modern units of the Russian fleet were the Borodino's (great name, terrible ships, like the worst of their generation) they were the only truely modern units with equal guns to the IJN's. The other ships were mostly older vessels with older guns with weaker shells, lower muzzle velocities etc etc. And because these guns were older types they lacked the range of the IJN's ships.
The IJN also took Captain Scott's love of long range gunnery and really ran with it, smart considering they guessed they would be outumbered. In 1906 they were firing at about 8000 yards at the most. In 1899 - 1900 that range is simply unthinkable. Maybe, MAYBE 4000 yards at the most, but everyone for the most part trained to fight at 2000 yards OR LESS (the RN was still training its men to use the Cutlass for gods sake :s ) and at those kind of ranges, the guns of the time WILL penetrate armour.