nothing harmfull,,,wipe dalvik will erase dalvik memory & cash(sometimes when installing roms,there are data left in da dalvik mmry which v dnt need after installing,and they cause trouble sometimes),,dalvik is more like jvm,,
as the da name says,, batttry wipe will erase all the battery stats,,,,
dont worry,,,it wont break your phone,,,,

,,,,but the data will be lost,, and also,,,first boot may take long time(~5 mints),,dnt wory,,,
BUT,,make sure you have enough juice left in your device b4 doing any of those things,,(i would prefer more than 40%,,its enough if you have some thing around 20%-30%,,yet just 2 b in da safe side,charge your battery.... B) )