When Glowing Fish says don't do it, he means it. You will usually find DXM in cough syrup, and most brands contain Acetaminophen, Guaifenesin, or Chlorpheniramine Maleate. Considering the usual 3mg/ml (if I remember correctly) concentration of DXM, the required 100ml of syrup will likely contain enough of the abovementioned substances to inflict serious, likely lethal damage lest you chose the brand carefully. Not to mention that the suggested 300mg dose of DXM is fairly high for "recreational" use.
Diphenhydramine is another fun one. This is something that is occasionally used as an easily availible and presumably relatively pleasant means of suicide. I'm not sure that it's particularly effective, although >800mg or so might just do it.
Now, I am not aware of synergetic effects between the drugs mentioned, but the effects are unlikely to be simply cumulative like described above. The best outcome would be vomiting immediately, and I suspect this is pretty likely to happen due to the combination of DXM and alcohol. Otherwise death or cognitive impairment is my best guess.
Seriously, don't do this. Try some friendlier drugs if you'd like to examine your subconcious.
Not that I'm endorsing any drug use at all.
Glowing Fish points out that DXM tablets were indeed suggested by the original writeup. I assumed those wouldn't be readily availible, which doesn't seem to be the case. Shows how much I know about modern drug culture.