Quoting Network World

In last week's column, you make a point about the cost of a test network not being cheap. You didn't mention the availability of virtual server software from VMWare and Microsoft that allow multiple virtual machines to be set up on a single workstation or server. This can significantly reduce the hardware and, with snapshots (VMWare), provide testing, quick "un-dos" and standby setups of servers.
-- Arnold.

You make a good point about using server virtualization software instead of multiple systems to construct a test network. There is definitely a cost savings to be had in this situation since you could theoretically run an entire test configuration on a single computer or maybe two with sufficient memory installed. Server virtualization provides for the ability to "move" a particular server's functionality between servers with minimal work. Having virtualization as a part of your disaster recovery plan is a tool that could help you get critical services running quickly until you can get additional hardware in place to run it the way that you would prefer.
 
The companies that I talked to for last week's column went with the individual server approach instead of virtualization for a combination of reasons.

One company in particular chose the individual server approach due to the database and application server system that they were using. The database and application servers had memory requirements large enough that they couldnt get enough systems combined onto one platform to make it worth the effort. This same company also encountered resistance from the primary vendor to support this configuration even in a test situation. Granted, this shouldn't be a problem but it is important that all the vendors involved in the products to be run under virtualization be willing to work with you even on a test configuration. 

Another company went with the individual system approach because they felt more comfortable with this as it would let them quickly put a test system into production when a primary system failed or needed to have maintenance performed on it.

Both virtualization and conventional duplicate servers have a place in test networks. In some cases, virtualization will be the more economical approach to go with. When there is reluctance on the part of a vendor to work with you on this, using duplicate servers may be your only choice for a test network or to have something ready when an important system fails.

Read the original, here.