I read Ian Jane's DVD review of Robocop Trilogy at http://www.dvdtalk.com/reviews/read.php?ID=10692 and......"Robocop 2 was filmed in Houston, not Dallas. Much of the footage was shot in downtown Houston, as many streets were closed off at night for the production. The outside street level and lobby of the OCP building is actually the opera house, the Wortham Theater."

Much of "Detroit" in Robocop 1 was filmed in Pittsburgh, I believe Dallas was used for most of the exterior action - but a lot of the interior stuff, plus the big climax at the end of the picture at the abandonded mill was filmed in Pittsburgh, PA.

http://imdb.com/title/tt0093870/locations

Robocop 2 was filmed entirely in Houston.

http://imdb.com/title/tt0100502/locations

Robocop 3 was filmed in Atlanta.

http://imdb.com/title/tt0107978/locations