The paltry 12MB sized update takes the phone from version I897UCJF6 to I897UCJH7 and features media scanning time improvements, improved calling experience, Exchange 2003 support, enables QuickOffice for Android users to edit Office documents, and some other messaging enhancements.
AT&T is apparently rolling it out in batches of 15,000 through Friday, but it appears that you'll be in for a surprise if you attempt to manually check for the update. If you happen to be impatient and attempt to do so, it'll reply back to your request saying that it's not yet ready and you'll be forced to wait 24 hours before trying for another request. So maybe you might want to just wait until that notification hits your phone?
Samsung Captivate Specifications | Review
source: AT&T & Android Central