Are you using DBAN or Eraser? DBAN is normally what you would use to erase a whole drive. Eraser is more aimed at particular files and unused space.
If you are nuking the drive, there is no reason to be concerned with a crash by interrputing the erase process, since you are overwriting everything anyway. If the drive is not having physical problems already, then you should be able to restart the PC and treat the drive as unformatted.
The only reason to not interrupt the erasing process is that previous data would not be overwritten. But 8 days for a 20GB drive seems really odd. Did you manually set the number of passes, or did you take one of the defaults? It would normally be done by now, even with a slower Pentium.