SQL server will take as much memory as you can give it.
For performance I recommend:
1 virtual machine for scanning, 4 virtual processors (or more), memory is not that important 1 virtual machine for sqlserver, 4 virtual processors (or more), 2GB ram (or more if you have more) Setting the logging mode for the database to "simple"
Perhaps the performace issues could be related to the 2005 SQL server running on they same VM. Are they known issues with running both together on the same box for 30,000+ assets?