Here is some tips that I've been gathering for using SharePoint 2010 Virtual Machines:
External USB 2.0 are too slow, you shoud use e-Sata, with 7200 RPM, or SSD
The VM virtual discs shoud have a unique file with fixed size, when you create VM virtual discs you shoud format it with 64KB in NTFS
The SQL Server tempDB shoud have two data files, if possible each on different discs, with a start size of 500~1GB and with 256MB for incremental growth
All SharePoint databases shoud have a start size of 500~1GB and an incremental growth of 100MB or higher
If you are using Sql Server 2008 Enterprise on your virtual machines please enable the Lock Pages in Memory option, see here how to do this, if you are using the Standard Edition you have to install SP1 and SP1 CU2 to use this option.
Right now i'm not using Virtual Machines i have SharePoint installed in my Windows 7 host, but this tips should also be applied in this scenario, keep in mind this is just a standalone environment for developers, it's not meant for production at all.
If you want to install SharePoint 2010 on Windows 7 here is a link to Harbar Tips and tricks for running SharePoint Server 2010 on Windows 7 and also on msdn Setting Up the Development Environment for SharePoint Server.
