Multiple Drupal site on one server?

Hi there,
We've got a powerful dedicated server with the following specs:
• Single Socket Dual Core Opteron 2214 2.2 GHz
• Red Hat Enterprise Linux 5 - 64 bit
• 4 GB HP RAM
• 146GB SAS 10K RPM Drive x 3 in RAID 5

Will it be Ok to run 20 low traffic Drupal site on this one server? What possible problems could we run into?

Thanks,