for online backup, if u r going to use replication, then u need huge amount of RAM, by which ur application, queries, and so on... and even live backup takes place.

so look at it if have enough amount of RAM, my opinion is 8GB RAM will be fair for this task.

but why don't u a shell script or use any user tweakable source application in which the process is like this

1) first it make dump compressed with tar.gz then transfer to storage.
ofcourse it will also take much memory but with less bandwidth.