What happens with EFS-encrypted files, if I install Windows 7?

Not unless you backup your key!

Instructions:

1. Click Start
2. Click Run
3. Enter 'mmc.exe' and click OK
4. Click File, then Add/Remove Snap-in
5. Click Add
6. Select Certificates and click Add
7. Leave selection on 'My user account' and click Finish
8. Click Close
9. Click OK
   10. Select Certificates - Current User in the lefthand pane of the MMC console
   11. Select Personal
   12. Select Certificates. Your personal certificate information should appear in the righthand pane of the MMC console
   13. Right-click on your certificate and select All Tasks
   14. Click Export
   15. On the Welcome screen, click Next
   16. Select 'Yes, export the private key' and click Next
   17. Leave the defaults on the Export File Format screen and click Next
   18. Enter a strong password, then re-enter it in the Confirm Password box, then click Next
   19. Enter a name to save your EFS certificate export file and browse to choose a destination folder to save it in, then click Save
   20. Click Next
   21. Click Finish

Then import it on your windows 7 installation:

1. Right click the .pfx file that you exported and press Install PFX.
2. Press Next and confirm you have the correct file.
3. Press Next and enter the password for the private key.
4. Press Next and press Place all certificates in the following store and press Browse.
5. Press Personal, OK, and Finish.
6. Press Yes and OK.

Yes you will, as this Technet article explains:

EFS in Windows 7 supports a "mixed-mode" operation of ECC and RSA algorithms. This provides backward compatibility with EFS files that were created by using algorithms supported in previous versions of Windows. This might be useful in organizations that use RSA and also want to use the ECC algorithm to prepare for Suite B compliance.