If you want longevity and max safty, go with steel sprockets, and 530 chain. Dont scrimp here unless you really know what you're doing. You don't want that sucker to fail. 530 means a wider, stronger, heavier chain. That's what's stock. About 1 to 1.2 pounds difference in a 520 and a 530. That may sound like a lot, but I think most people would say you cant really feel the difference.
Chains dont came assembled. You couldn't get it through the swingarm if they were. Get a good quality chaintool, upwards of $50. If you've never done any kind of chain at all before, then consider a professional do it for you. Crimping the rivits right is soooooo important.
As far as gear ratio choices, 16/40 is stock. What's your riding style and how much do you wiegh? I choose spirited, 15/42. It gives quicker acceleration, but at the cost of top speed. It gives you what feels like more power if you're a heavier rider. 15/41 is common too. I've even considered a 15/43 to get just a little quicker launch, and shorter shiftpoints. I've heard someone here running that ratio.
SubSailor has lots of technical info in his post, so if you want to research it, start with the stuff he put on his bike.
My 02 cents..
John, 2000 RC51 #000100