We collect ~6GB a day from 3 firewalls with informational logging via syslog. The current environment, which we are still testing, is a 1TB server with Kiwi Syslog daemon into a MS SQL database. This is relatively easy and can be setup with little time.
The challenge is setting up the frontend, we have basically taken PhpSyslogNG which is an open source GPL licensed product and used that template to create our own custom front end on a .NET platform. The result is a completely searchable database for all syslog messages that can be sorted/searched via any field (date, logging level, device, etc).
Hope this helps, here are some links for ya..
http://www.vermeer.org/projects/php-syslog-ng
http://www.kiwisyslog.com/info_syslog.htm
