I think its not an expansion.. anyway you can get AC2 installed in 2 days (depends on net).. (If you dont wanna buy the original).
BUT as i said, i dont think its expansion. I just downloaded AC2 its awesome. AC1 i kinda boring all the same places, houses where you kill, AC2 has unique main quests + new stuff makes it more interesting.... So I think ACBH will be something -beep-ing awesome :DD
<Edit1> PS AC1 has all the same small quests like pickpocketing, beating up, listening from a chair etc.. :PP