The capacity planning and management center is up and running. It has a ton of great documents on it, it's very exciting! Check it out:
Here's a quick look at the documents I found most useful:
A great overview document describing the process you should go through the planning exercise for your SharePoint 2010 environment, whether that is a brand new environment or an upgrade, or just if you need to learn how to evaluate if sharepoint is the right solution for you.
The software boundaries and limits document, which lists all the limits that you might care about when considering SharePoint 2010 as the solution for your organization.
Technical case studies - these are case studies that document a lot of information about the live environments that use SharePoint 2010 within Microsoft.
Storage and SQL Server capacity planning document - this is a great place to go if you want to learn how to plan for and manager your SQL Server with SharePoint 2010
If you share your feedback on the documents with me, I'll make sure to pass it along to the authors, so please feel free to comment :-)
I found that useful tool, made by HP :
HP Sizer for Microsoft SharePoint 2010, I present it on my blog if you are interested :