It'll probably knock you out, it's unlikely it will "get you high".
Well I know that benny's can get you off your ass, a kid in my school got expelled for taking 500mg at school. I just prefer being safe.
You will experience some sedation at that dosage level. I don't think it'd provide noticeable relief for coming down from a 'long amphetamine binge' (which I take to you mean you've been up for at least several days, possibly dosing daily longer than that). It wouldn't hurt though.
I'd eat a full meal, if you haven't yet, that can help. Personally, I like getting good and drunk to help with comedowns/pass out, but watch out for dehydration/overdoing it (and it's not too healthy). Of course benzos (or other GABAergics)/opiates are preferable, some like using neuroleptics such as quetiapine, and cannabis has its fans as well.
Changed title to reflect your actual question, and moving to BDD since we're talking about amp comedowns and not recreational use of diphenhydramine.
PD->BDD
Off your ass to a psych ward in delirium.
50mg of benadryl is the typical max recommended dose, there shouldn't be any adverse side effects that I know of. I'd recommend the OP get his hands on some benzos for the phet comedowns, rather than experiencing the distress he exhibited today online...or just stop abusing adderal.
Trazodone is a great drug (oftentimes prescribed for insomnia in lieu of the extremely dangerous benzodiazepines and [not] well-named "non-benzodiazepines"). It knocks most people out, and did me last night.
That said, I'm pretty sure trazodone doesn't have NRI properties, or if it does, they're negligible. It is an SSRI, but is only really active at doses in the hundreds of mgs.