Right, here goes.
OFP1 was deveoped by Bohemia Interactive and produced by Codemasters. They eventually fell out and went their seperate ways.
BIS created the Arma series as a spiritual predecessor to OFP, while Codemasters are making OFP2, nothing more than a franchise sequel (though it looks promising).
Armed Assault was pap. AI was s**te, campaign was pony and the whole game felt very incomplete. Arma 2 is supposedly going to be alot better, with a better campaign and alot of new features, but after Arma, I wouldnt hold my breath.
Yes, its PC only and afaik, youll need a pretty respectible rig to run it.
Hope thats cleared everything up!