2
According to INSTALL.html, which I think is correct, uploads/, cache/ and templates_c/ and the file mainfile.php need to be writable. So typically those directories would need 777, and mainfile.php would need 666. After completing the install, mailfile.php should be changed to 644 or 444 (so that it's not writable by the script).
As I recall, the XOOPS install script was smart enough to tell you if the permissions were wrong.