Refer to page 1-11 in your Manual.
Note also, that these units are shipping with 3GB RAM as they most probably have Windows 7 32bit installed. There is little point in upgrading to 4GB if you will continue to use W7 32bit as only 3.2GB-3.5GB will be available due to 32bit addressing limitations. And there is little point in upgrading to W7 64bit, just to be able to fully utilize the 4GB unless you have 64bit programs to use, as 32bit software will still face the same limitation.
To sum up: 3GB is plenty memory for most uses, so if you're happy with the unit as it is, then just leave it and enjoy using it!