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This is a question I am asked repeatedly. Unfortunately, there is no absolute answer as not all applications are created and hosted equally. Some of our customers run 40 vBlades on a single Processing Blade, while others run just five. Both are equally happy with performance and seeing great return on their investment. However, here are few things to consider and some tips to help you size and architect your VMs:

1. Track the CPU and memory utilization of all your servers - make sure to log it by time of day and type of day. For example, applications involved with equity trading will have high utilization when the market is in session.
2. Determine the uptime requirement for the application. Does it require N+1 hardware high availability? Are you OK grouping together mixed application VMs where some require little uptime and others require a lot?
3. Figure out if your application requires chargeback. Is it SAN hosted? Are you using VLANs for your applications? (SAN and VLANS are key factors to processor virtualization as they facilitate easy VM moves from bare metal to bare metal)
4. Are there security policies in place that require physical separation of servers via physical firewalls or switches?
5. Can applications that are IPv6 compliant live on the same machines that are IPv4?
6. Can Linux operating systems coexist with Windows operating systems?
7. Do any of your applications require access to physical devices (like USB) on the metal?
8. Consider moving from Dual-Core to Quad-Core x86 blades. Your power needs will remain the same but you’ll be able to increase the number of vBlades you can host safely by 35%. Not to mention Intel and AMD are embedding virtualization triggers into most of their chips going forward.
9. Use assess tools like Platespin's PowerRecon. This is one example of a good tool that can help you navigate through the sizing effort.
10. Inject tools into your architecture that allow for easy migration of VMs from one bare metal server to another. The tools allow for re-sizing of VMs (CPU and Memory) based on load. The tools also enable virtual to physical migration. For example, Egenera PAN Manager allows you to run your "server" on a vBlade and then host it on a physical "pBlade" if needed later. Going from p 2 v and v 2 p is critical, as your Windows file server today may not need a dedicated bare metal blade but when you grow 5x (say because of an acquisition), it may very well need to.

The bottom line here is to think about the plan first, engage subject matter experts and invest wisely. Take the time to assess your architecture as it’s very easy to get caught up in the great excitement that is virtualization. Now that VMware has gone public and XenSource will be merging with Citrix, the hypervisor market is becoming larger than ever. Platespin has done a nice job developing and marketing their tool to take advantage of this trend as has Egenera with PAN Manager software. Of course at Egenera, we're extending the value of the hypervisor through data center virtualization. The hypervisor is just one small piece of the puzzle.