Alright.

First of all, 600 mg is bound to send you straight to third-plateau and back. You must keep in mind that as the dosage increases, the character of the trip increases drastically. If you do reach third, it's not going to be the same buzz you got X2. It's going to feel like a completely different drug.

http://www.third-plateau.org/faq/dxm....shtml#toc.5.7

All this in mind, if you are still confident, go for it. Just try not to make a habit out of DXM. I've been there, and everyday life starts to become one big trip. It's contentment in a bottle. Olney's Lesions, a theory about brain damage caused by dissociatives, has not been proven in any human cases, but it hasn't been disproved either. Play it safe.

http://en.wikipedia.org/wiki/Olney's_lesions

Don't eat before tripping, unless you're going to eat very light. I can't stress enough the potential for losing the trip eating has.