I'm looking for a data store that is very memory efficient while still allowing many object changes per second and disregarding ACID compliance for the last X records.

I need this database for a server with not much memory and I can make a key-value store, document, or SQL database work.

The idea is that indexes/keys are the only thing I need in memory and all the actual values/objects/rows can be saved on disk do to the low read rate (I just want index/key lookup to be fast). I also don't want records constantly being flushed to disk, so I would like the last X number of records to be held in memory so that 100 or so of them can all be written at once. I don't care if I lose the last 10 seconds worth of objects/values. I do care if the database as a whole is in danger of becoming corrupt.

Is there a data-store like this?