First: Remove those 777 permissions. You need this only in cases with conflicting ownership. Try to run PHP as FastCGI – for example per .htaccess:
AddHandler php-cgi .php
# or
AddHandler php-fastcgi .php
Set up a notification mail for every 404 request. You will be surprised how many attacks the average blog gets each day. Wait … I just published my 404 notifier on GitHub:
Plugin T5 404 Tools repository · Download latest version
Also, install a plugin to prevent password guessing. I use Login LockDown with a lockout length of 45000 minutes.
Read your log files regularly. There is no better way to get all critical information.
There are also plugins which check all files for changes. I had just problems with those: slow, a lot of misleading information, just too much noise. But if your site hasn’t that many files it may be an option. Test it.
Update
One note about file permissions: Usually you don’t want anybody to send DELETE or PUT requests to your site, especially when something is set to 777. 😉
To restrict the allowed request methods to HEAD, GET and POST, add this to your .htaccess:
<LimitExcept HEAD GET POST>
order deny,allow
deny from all
</LimitExcept>