How Much Memory Can Be Installed In A MacBook Air?
The current version of the MacBook Air (2010) supports a maximum of 4GB of RAM (2GB is standard). Memory cannot be upgraded later with the MacBook Air since the RAM is soldered to the logic board, so any memory upgrades must be purchased with the laptop. Luckily, App Sliced partners like MacConnection, MacMall, and B&H all offer versions of the MacBook Air with upgraded RAM.
UPDATED JUNE 2012: The latest MacBook Air (mid-2012) supports a maximum of 8GB of RAM (4GB is standard). 8GB is a built-to-order option that has to be purchased with the MacBook Air. It's not possible to upgrade the memory after the fact.