Hi,
given that the unit can be charged from USB, I'd guess that you are safe with 5 V and 500 mA. I believe the stock charger is rated 5V/900mA, so that should really suffice.
I guess that the iRiver always draws the power from the battery, so that acts as a buffer if the device should need peaks of power (writing to disk for example).....
HtH,
Frank