Yes, it supports VST plug-ins (has an integrated adapter that works well).
It has effect plug-ins that come with it. The Sonitus EQ is integrated as a track EQ and you can create templates to automatically load other effects into new projects.
Are the virtual instruments that come with Sonar any good, or are virtual instruments in general any good?
The ones that come with Sonar are OK, but I personally don't use them. I have other virtual instruments that I prefer to use which are excellent.
Download the Sonar 4 Producer trial version and try it out.