One very easy win to gain fairly significant extra performance from your Virtual Server 2005 guest Virtual Machines is to use virtual SCSI disks instead of virtual IDE disks. Of course, a physical fast I/O subsystem on the host is going to make a far bigger difference, but you're probably not likely to be able to do that "on the cheap" (especially in a test/dev situation).
Lets assume you started with a Windows Server 2003 virtual machine, for example, and built it originally using Virtual PC 2004 for example. Virtual PC 2004 does not support virtual SCSI disks or virtual SCSI adapters, so the virtual disk in your machine will be IDE connected. This equally as well applies if you built a VM using default settings under Virtual Server.
If everything goes OK, your VM will boot as normal with no impact, except that you'll have better disk I/O performance. If you're happy with the changes, you can re-update boot.ini to remove the old IDE boot option, and change the default option in this file to match. Bear in mind that you will no longer be able to revert this virtual machine to Virtual PC 2004 without backing out these changes. I know someone will ask how much performance gain you get. There aren't any currently any official statistics I know of, although the Virtual Machine Program Manager, Ben Armstrong has previously stated that the main advantage of using SCSI virtual disks over IDE is that it runs the optimised SCSI driver as part of the VM additions. This in turn avoids a lot of the emulation overhead.