Make sure to get Robotussin without any acetomenaphine or other additives in it. These are usually the maximum strength ones, specifically for a cough. Vicks44 usually has more DXM, tastes better, and if you get the normal variety, no additives. Just check the active/inactive ingredients on the back of the package. And then check it again.
And I too have never heard "robo-baked". It does have a nice ring to it, but I think 98% of the industrialized English-speaking world calls the experience "Robo-tripping."
I haven't done this for QUITE a long time, but as I remember, two bottles is good. I've done three and there was little noticeable difference between two and three (just more confusion, rather than intensified visual or mental effects).
Oh, and DXM is not a hallucinogen! It's a disassociative. Completely different; the visual effects you experience are your brain misinterpreting the visual signals, rather than any actual hallucination or visual illusion. DXM is relatively safe if you use a source without additives, but it does have its downsides. On a related note, having sex on DXM is... definately interesting.
~Ethno