I just turned on VDBD now and the Task Manager shows it's using between 40 to 70% of my ram. That's why I turned it off.
I also use avast and love it. I have VRDB doing its thing when the screen saver comes on.
The following is a copy of how avast describes the principke and settings of VRDB:
VRDB PRINCIPLE
avast! creates an integrity database, i.e. it stores information about the actual state of the files, doing it three versions back for each file. The database creation/maintenance is performed either when the computer is idle, or when the screen-saver is running (any screen-saver, not only the avast! one). This database, once it is created, is updated each three weeks (this value may be changed by editing avast4.ini).
If a file is infected by a virus, it may be possible to repair it using the information stored in VRDB, i.e. turn it to its original state. If there are multiple versions of the file in the database, you can choose which version you want to restore.
SETTINGS
The settings of VRDB can be changed by clicking on the icon in the system tray with the right mouse button. There are three options:
Generate VRDB when computer is idle. avast! will create its database only when the computer is idle, i.e. when it is not being used.
Generate VRDB only when screen-saver is running. avast! will create its database during the time screen-saver is running. It may be any screen-saver, not only the one included in avast!
Disable VRDB generation. avast! will neither create, nor update the database. If you select this option, it will not be possible to repair virus infected files in the future!