Upgrade Gentoo using another box
Sync your portage tree
On your high bandwidth machine, download a recent snapshot here: ftp://ibiblio.org/pub/Linux/distributions/gentoo/snapshots/
Copy it onto your bandwidth challenged Gentoo box and untar it into /usr/portage.
Get the links to your distfile upgrades
# emerge -uDp --fetchonly world 2> /tmp/distfiles.txt
Download the files on the high bandwidth box
This step depends entirely on the OS you use. Use any tool you have at your disposal. A good download manager or Cygwin running on an XP box works fine.
Upgrade your Gentoo box
Copy the tarballs you got into /usr/portage/distfiles/ then do a
# emerge -uD world
Other solutions
# get required URLs
emerge --pretend --fetchonly --update world 2> list.txt
# convert URL list to wget format
cat list.txt | sed 's/\shttp:/\nhttp:/gi' | sed 's/\sftp:/\nftp:/gi' > wgetlist.txt
# get 'em
wget -i wgetlist.txt -nc
