As Mr. Blonde said, don't mix it with tramadol or u'll get serotonin syndrome. I weigh about 78kg and I never really felt DXM until I took at least 600mg, but then again it takes a lot of most substances to get me going. In my opinion DXM sucks without smoking bud, so I would definitely recommend having a bong around to toke on during the high if ur into that kind of thing. Have fun.
Edit: Also I forgot to say make sure ur in a comfortable environment with no parents around while you do it or it can really ruin the trip. It often makes me too scared to go to the toilet cause I'm worried one of my folks will see me, so I only do it when away.