Alot of people have been using their palm pre chargers, which are 1 amp and haven't had problem. Its under alot of debate, but no one has reported problems and it seems what people (who seem to know more then me) say is the worst case scenario is that the lifetime of a battery could be shortened, but not likely to damage the phone itself. I am far from an expert, just a summary of what i have read here but i have used my 1 amp pre charger without issues so far.
We know that the charger shipped with the Epic is rated to deliver 750ma. We must then conclude that the charging circuitry in the phone is designed to draw 750ma or less.
Having a charger with 800ma or a full 1 amp, or heck even a billion amps CAPACITY does not mean that much is forced down into the phone. It just means the charger has that much CAPACITY. The charging circuits in the phone will still attempt to draw 750ma or less.
You ought not worry about using a charger that's over-qualified for the job. The thing you ought to be careful about is using any charger that's under-qualified.
Edit: Well I've been schooled. The above would be correct, except that the bolded part is a bad assumption. The phone will indeed allow itself to draw more amperage than the shipped charger is capable of delivering. So forget everything I said.
I see a few people are saying they are using their Pre chargers with their Epic. I have a Sprint Pre and an Epic 4g and my Pre charger will not work with my Epic. I did use a "Moment" charger that I borrowed a charge off of and it seemed to work well but I don't know how the Pre's charger could work unless they're using a Pre Plus or something....
It's not the MA you must worry about its the Voltage. You can have 10000000000000000000000000 MA and will be fine, its just the amount of draw the adapter can handle. Also did a quick Google Search... AC Adapters, Which mA Rating? - Yahoo! Answers
(this answer was voted high meaning it's more likely accurate)