A B737-300 experienced a NMAC with a Cessna flying through the approach path to the airport.

Narrative

We were descending from 5000 FT to 3000 FT; turning base to final. [We] had re-entered Class B airspace about 1 minute earlier. We reported the runway and an A330 in sight ahead of us then cleared for a visual approach. As we were descending traffic was reported at our 1 o'clock 3000 FT climbing; unconfirmed. We noticed the climbing traffic on TCAS but had no visual yet. As it appeared that he was almost on the localizer I started a turn to the left off the localizer and stopped the descent at about 3500 FT. The First Officer stated traffic in sight at about the same time a TCAS RA sounded a 'climb; climb' advisory and indicated a 2000 FPM climb. I disconnected the autopilot and autothrottles and initiated a climb. 'Clear of conflict' was announced and I leveled off at about 4200 FT. We told the Controller about the RA and climb; then resumed the approach and landed uneventfully. I never saw the other aircraft but the First Officer said it appeared to be a Cessna. Several passengers told me after the flight that it appeared that the Cessna was about 100-200 FT off to the right and only 100 FT below us. One passenger told me that it was so close that he could 'identify the other pilot in a line-up if necessary.