If your goin the CGI route bear in mind that the difficult bit when mixing real footge with CGI is motion tracking, synching your real camera's motion with your CGI camera motion. The more complex your camera pans, tilts, tracks etc the harder it becomes to get your footage to match up.
Big budget- get a motion control camera rig!
Low budget- it can be done but youll need a decent 3D compositor like shake or combustion as well as modelling SW (Maya, Max et al). Carefully plan your shot and try and keep things simple!

tonga