... I need to start downloading soon as there's a lot.
The repository you mention is not recommended by the dev's, as it is a primary repository used to sync other mirrors. Too much usage by the general public will overload the server. Check your Synaptic install for other repositories to use (also, /etc/apt/sources.list). It is unclear to me exactly how you plan to do the downloading ... by broadband connection or dial-up. The prospect of downloading up-dates via dial-up connection is a daunting task, one that I would not consider attempting.
You can generate a list of your installed packages with this command:
rpm -qa --qf '%{name}-%{version}-%{release}-%{arch}.rpm ' > installed.txt
Only Synaptic, however, will tell you what packages can be up-dated. I would never recommend that Synaptic be by-passed for package installation. Without it, the procedure is just too complicated.