Power consumption estimate

I was wondering if you guys could help we figure out a good guess about power consumption. i have an arduino decimila hooked up to a vinculum Vdrive2 with a usb thumb drive. also the arduino board is monitoring a reed switch and a hall effect sensor. i need to know how amp hours i'm going to need to run this setup for a year(worst case). i open a file on the thumb drive, write 25 bytes and close the file once per minute(worst case). Also, should i be looking at a 9 volt battery or a 12volt battery?

Thanks

Jeff O'Brien