Handbook:AMD64/Blocks/HWReqs/en

! Memory ! Disk space ! Swap space
 * 256 MB
 * 512 MB
 * colspan=2|2.5 GB (excluding swap space)
 * colspan=2|At least 256 MB
 * }

The Gentoo AMD64 project page is a good place to be for more information about Gentoo's support.