OK, for the non-medical folks here...
-81 mg of aspirin (aka "baby dose") is the normal once-daily variety. Going higher in dose hasn't shown any improved efficiency. The prevention benefits are identical.

-During a heart attack, 325 mg (aka, 4 baby aspirins) is the dosage. However, the hospital is going to put a lot more meds in you: oxygen, morphine, nitro, heparin, etc. Aspirin just buys a little time.

-Allergic reactions can't, by definition, occur until you've had at least one contact with the offensive agent. That is, most people aren't allergic to bees/peanuts/shellfish/etc the first time. Your body has to recognize the offending object, and over-reacts to it using pre-made antibodies. You can't make an antibody until you've been exposed to something once (just like no one is immune to chicken pox without having first caught it, or had the vaccine!)

-Aspirin probably is worth the weight of carrying. Just don't expect it to stop chest pain by itself.