you can Register a photo or group of photos with the US Copyright office electronically (much better than by snail mail).
the only limitation on # of photos is total upload time of (IIRC) one hour.
photos in a group have to be either all published or all unpublished (having them on a website= published). if they're published photos, they have to have all been published within a calendar year to be included in one group (called a collection).
you also do not need to send the full sized version. a 100kb version is fine.
so, you can register a group of photos called "2009 collection" by zipping 600 photos into 2-3 zip files and uploading the zip files to the .gov site.
if none are published, you can do a collection called "1990-2009 collection" and upload ALL of the photos you want registered in one batch (keep the Zip files under 6 MB each. you can upload any number of zip files under a collection name).
takes a month or two to get the Registration certificate.
here's the .gov e-copyright site:
ECO IIRC, the cost is $35 per collection.
once you get all your photos organized and Zipped, create a username, log in, and it'll walk you through. each "collection" is assigned a case number.
just be sure you're using a fast connection when you go to upload as there is a time-out of about an hour as I recall.