OS X Mountain Lion will be available on Wednesday, July 25th from the Mac App Store for US$19.99.
Mountain Lion requires Lion or Snow Leopard (OS X 10.6.8 or later), 2GB of memory and 8GB of available space. For a complete list of system requirements and compatible systems, visit: apple.com/osx/specs/. OS X Server requires Mountain Lion and will also be available on Wednesday, July 25th from the Mac App Store for $19.99.
The OS X Mountain Lion Up-to-Date upgrade is available at no additional charge via the Mac App Store to all customers who purchased a qualifying new Mac system from Apple or an Apple Authorized Reseller on or after June 11, 2012.