Depends on what you want the 2821 for?
Files include the following:
1. IOS (c2800nm-advipservicesk9-mz.124-3f.bin);
2. SDM;
3. Cisco IP Tel files;
4. UC files
If you want to only use this router for non-VoIP, I suggest you back-up all the files and leave only the IOS behind.