From the world of NoSQL, let me nitpick a bit: You don't need to run ACID software to do safe processing of your transactions (durability and clever business-logic constraints can serve to limit your exposure to losses to finite knowable values, which may be worth it because you can achieve availability and partition-tolerance)... but man, ACID sure makes things a heck of a lot easier, and will save you from many classes of implementation error. Think twice before giving it up. Think more than twice.
(Also, MongoDB is like the worst NoSQL data store out there for a task like this.)
(Also, MongoDB is like the worst NoSQL data store out there for a task like this.)