let's see, where to begin...
first, there's the whole um, antidex campaign that's running rampant on here. what's the point in being down on dxm? yeah, you can buy it at any pharmacy. so what? it's still highly psychoactive and extremely dissociative at higher doses. trust me on this one. or, just trust anybody who has had a high level trip.
as for the thing about feeling dirty the next day, that's an individual thing, i think. not once have i had the feeling that dexing was dirty. every time i do it, i get new insight on something that just blows my mind. on this note, andx, 500mg is nothing. 500mg makes me bouncy and i consider that a medium dose at best. try a gram sometime. you'll reconsider its value as a "real drug." a gram of dex would put you beyond outer space, more than likely. but, hey, what do i know, right?
anyway, back to damian's original post. i wouldn't buy anything that had guafinnesin in it. it'll make you puke your guts up, which isn't good for getting the dxm into your system. try going to rite aid, walgreens, cvs, or any other pharmacy and look for generic tussin max strength cough. it's quite a bit cheaper than robo (

z of generitussin is only a buck more than 4oz of robo). and, like flag said, start out low. read william white's faq on erowid if you haven't already. it'll give you a good idea of what to expect. don't expect the lower doses to be anything spectacular, because they're not (personal opinion). but once you start hitting the doses where you lie down and everything falls apart around you and you're left to rebuild reality, that's when it gets fun. real fun.
and something else. you can ignore any of the negative posts about dxm in this forum. going back to flag again, it's just the people being close-minded. but, only 1/3 of the population, according to the faq, enjoys a dxm trip. the people who post negatively about it do so because they didn't enjoy it, or it didn't get them as messed up as they wanted it to. doesn't give them any right to condemn others for using it or wanting to try.
good luck with your first try, but get something besides dm, cuz it's trash with guafinnesin in it. and whatever you do, stay away from coricidin. hate to see people die from od'ing on the other stuff. nyquil's bad, too. has acetaminophen in it, and it only takes 4g to be fatal. anyway, good luck again kid, and be sure to post a report when you try.