I believe that was George Noory. If you recall, Art tried to retire once, but couldn't, because he hadn't properly grooved in a competent replacement.
From what I remember, he'd had George fill in for him several times already, so George took over full time during Art's first abortive attempt to leave. I think I'm recalling that episode correctly, but it was about ten years ago, so my memory of it may be off a bit.
I distinctly remember when George officially took over after Art's final departure. The man was so fake and wooden, I could hardly stand to listen to him. He got better (and more relaxed) over the years, but he's never matched Art's level of skill and ability behind the mic.
Mike Siegel replaced Bell in April of 2000. He lasted until February 2001. Siegel left when Bell returned to the show.