DXM isn't a flat-dosage drug, dosage is relative to your weight, and the desired effects. Depending on the mg ingested per kg of your body weight, VERY different effects can be achieved. 150mg is low regardless, but may be a good starting point for you. What kind if high are you looking for? Do you just want to test the waters, or get gonzo? Like previously mentioned, there is a good faq on erowid, or you can check out dextroverse.
Also, you should be forewarned that DXM is one of those drugs that there is typically no middle ground, you either love it or hate it, so chances are 50/50 that you could have a pretty damn unpleasant time, or vice versa.