My 'work' is at home (dialup). The local library is only good for about 1.5mbits/sec (about 150kbytes/sec). I don't have enough free disk space for a full repo on either my laptop or my desktop.

While not currently supported by the main CentOS project (AFAIK), there are presto repos out there for CentOS 5. Yum presto, using deltarpms, is the solution to your problem; OpenOffice.org and KDE, in particular, delta very well indeed; I've seen (on Fedora, which has presto enabled on the main repos) 90% reductions (that is, if an update set is 500MB, you only have to download 25MB of deltarpms) and even greater in updates of these. The savings in bandwidth are substantial, and I could not imagine tracking Fedora updates on dialup without presto.

See http://www.centos.org/modules/newbb/viewtopic.php?topic_id=8349