Registration is important -- how much extra would you pay for a stream fed press? Can someone explain stream feeding vs. push/pull when it comes to accurate registration?

Yes, stream feed generally allows you to run with accurate registration at higher press speeds than without - that's the idea. Which press(es) are you considering specifically?

Looking at AB Dick Century 3500S vs 3500, (maybe 3000) Ryobi 3302

Better go for a speedmaster. I also heard shinohara are great too. Ryobi is a middle quality to my opinion. I would get ABDick ever if they gave it for free. In general registration has nothing to do with the feeder (unless of course, the machine is not built well)

Stream feed v push or pull feeding, there is little difference when the press is young and the stock is good.

The main advantage as I see it, is the stream feed is forgiving when it comes to quality of stock it can run. If the sheet is curly then as long as you can feed the first 5 to 10 sheets the rest will just follow on without a problem. I have MO's here; the 4 colour is stream feed, the 1 & 2 colour have grippers and feed tables and the 4 colour can run stock the 2 colour has trouble with.

I agree. this is because the sheets cover each other half their length. I also want to add, envelopes are running faster with stream feed, especially the ones that open on the side.