In the midst of all the excitement of creating a new SharePoint 2010 farm or upgrade an existing SharePoint 2007 farm, capacity planning seem to be often ignored.
As Benjamin Franklin said “If you fail to plan, you are planning to fail”, so investing sometime on researching a bit about capacity management in SharePoint can save you a lot of agony in the future.
Following articles contain good details which can help you in planning.
Some interesting points to consider from Microsoft while planning SharePoint 2010 sites.
The following PDF lists some important Pros and Cons of creating a Site Collection vs a Site.