How you resolve this will depend largely on how your currently storing your documents  in SharePoint:
If your storing your documents across multiple Site Collections, then this is pretty simple. You can add new content  Databases to your Web Application via Central Admin and use this tool: www.microsoft.com/.../details.aspx to move Site Collections between databases so you offload some of the storage to the new DB.
If your storing your documents across multiple Sites under a single Site Collection, then it becomes a little more difficult because you need to move some of your Sites over to new Site Collections so that those Site Collections can be moved to new databases just as in option #1. This thread describes moving Sites to new Site Collections: social.technet.microsoft.com/.../...-920c3d2e2e15. One caveat of this is that it will change the URL schema for sites that get moved to new Site Collections. This can break content links and user's IE favorites. You can account for this and avoid the backlash using a tool I wrote to redirect the old URLs to the new URLs so user's favorites and content links continue to work. You can get that here: rdacollaboration.codeplex.com/.../Project...s.aspx
If your storing all your documents in a single location, or, for some reason, the 2 options above just won't work for you, then your last resort may be to look into Remote Blob Storage (RBS). This can allow you to store files outside of the content database  - such as on a file share. This article describes the concept: blogs.technet.com/.../...-in-sharepoint-2007.aspx. Also, if you visit CodePlex, search for 'Blob' and you'll see several related tools.
AvePoint has a product called 'Archiver' that implements RBS. I saw a demo once and it looked pretty nice. Here's their product page: www.avepoint.com/.../?en=apnl010901