It all depend... the A3000 is a better machine of the two both in stock specs and it can be uppgraded bewond the A1200 aswell. However both in buing and in uppgrading the A1200 can be a much cheaper alternative.
I would however look for an A4000 machine if I where you as the A3000 original case lack in space and have heat issues so it requiering a tower kit to take full advantage of the machine. Here the A4000 is better allowing for PPC board and PCI solusion and space for 5.25" unit without having to tower the beast.
To answerr the OS3.5 question, I belive it requier at least 4Mb of fastram to run.
