If the 2000 is anytihng like the 4k then they are current hogs from what i hear. You need another battery or two regardless anyways. You already got the alt so id pick up another battery for now. And then i guess it would depend on what ohm load you need cuz the ap does 3k @ 2 ohm and im assuming above is correct about the memphis doing full power at @ 1 ohm.
EDIT: Nevermind, if they both do full power @ 2 ohm then it depends on if you want/need that extra 1000 watts. Its prolly not even worth it, prolly wont tell much of a difference at all to the ear. Just get a battery for now is my suggestion.