Oracle recommends that you set swap space to 1.5 times the amount of RAM for systems with 2 GB of RAM or less. For systems with 2 GB to 16 GB RAM, use swap space equal to RAM. For systems with more than 16 GB RAM, use 16 GB of RAM for swap space.

http://download.oracle.com/docs/cd/E11882_01/install.112/e10812/typinstl.htm#CBHHHJJF