I ran across a cool article on Virtual PC Guy's Weblog that discussed the performance difference between using SCSI and IDE for your Virtual Server world.  Keep in mind that Virtual PC doesn't support SCSI drives so if you are planning on using the same drive across both Virtual PC and Server then you better use IDE.

Here is a summary of the key points:

As part of Virtual Machine Additions we install an accelerated SCSI driver.  Once this driver is installed the performance of our emulated SCSI controller is significantly faster than our emulated IDE controller.

And don't forget, initially creating your virtual disk as a SCSI drive on Virtual Server will use the slower emulation mode, causing your guest operating system installation to crawl compared to the IDE disk.  With R2, Microsoft adds the virtual floppy disk containing the SCSI Shunt Driver, which can be used by hitting "F6" during the Windows operating system installation to load the better performing SCSI emulation driver... and in return, speeding up your Windows guest installation.

If you have installed on IDE and want to move to SCSI what you need to do is:
1) Add a SCSI controller to your virtual machine
2) Boot the virtual machine
3) Uninstall and reinstall the Virtual Machine Additions
4) Shutdown the virtual machine
5) Disconnect the drive from the IDE controller and add it to the SCSI controller