I managed to see Spock's Beard open for them several times in 2000, and they were the first band I really enjoyed despite not knowing much of their material. Actually, seeing them live moved me to pick up their (then) new album V.
Exact same thing happened with me. Had never heard of SB, but was totally floored by the performance. Picked up V right after. Those were the days still when DT was more of a prog band than a metal band, so having SB open made sense.
Can I chime in to say it was for me that way also?
Didn't know Spock's Beard before but they blew me away, went the next day to purchase V and am a big fan ever since.
Others I saw:
Pain Of Salvation: didn't leave a good impression, were too chaotic, but I checked out their records nonetheless. They are great but they either had a bad night or it wasn't the right time for me to appreciate them.
Vanden Plas: Left me cold and I was kinda bored during their set, never did check them out later
Symphony X: Good, but had some sound issues, always meaning to go deeper but never had the time.
According to the tourography on mp.com I also saw Fates Warning, Enchant and Tiles but I have absolutely no memory of them, so either they didn't leave any kind of impression or the info in the tourography is wrong. I definitely wasn't late to the shows and at least Fates Warning was a band that I did know (if only casually) at the time so I would/should have enjoyed them.