As with most Microsoft products, every time there is a new version the hardware requirements to run the new software increases to keep up with evolutions in hardware and to provide more resources to the new features within the product.  SharePoint 2013 is no different…here is a table that shows the evolution of hardware requirements from from 2007 to 2013:

Component
2007
2010
2013
Web Front End
Dual processors that are each faster than 3 GHz
2 GB RAM
3 GB of free space plus adequate free space
64-bit, four cores
8 GB RAM for production use in a single server or multiple server farm
80 GB for system drive
64-bit, four cores
12 GB RAM for production use in a single server or multiple server farm
80 GB for system drive
Application Server
Dual processors that are each faster than 2.5 GHz
4 GB RAM
3 GB of free space
64-bit, four cores
8 GB RAM for production use in a single server or multiple server farm
80 GB for system drive
64-bit, four cores
12 GB RAM for production use in a single server or multiple server farm
80 GB for system drive
SQL Database
Pentium III-compatible processor or higher
Recommended: 1 GHz or higher
Recommended: 1 GB or more
64-bit, eight cores for medium deployments
8 GB RAM for small deployments
16 GB RAM for medium deployments
64-bit, eight cores for medium deployments
8 GB RAM for small deployments
16 GB RAM for medium deployments

For those of you looking to use existing hardware for SharePoint 2013, you may need additional hardware to keep up with the latest version…this could be another good reason to move to the cloud!