The only person that reacted to Quentin was Amy, and her reaction (being very frightened of him) tells us that she certainly remembers Quentin's ghost haunting Collinwood. So it's also not a stretch to think that David remembers as well
As mscbryk mentioned, David's reaction was captured in the recent slide show. He recognized Quentin's music ("Quentin's back!"), and after following it to the West Wing, he fled in terror when he realized a man was hiding in there.
I always thought it was a mistake for the present day characters to remember Quentin's haunting, but once history is changed by time travel, how DO you address the issue of a circular paradox except to have the change take place from the point the time travelers return? In a circular paradox, everything to date is changed due to the time travel, yet the motivation for visiting the past sprang from the original history.
Spoiler:In 1967, Vicki's trip through time changed nothing, thereby creating no circular paradox. In 1968, some characters recall what occurred at Collinwood before Barnabas altered Quentin's history, so Barnabas' earlier motivation to visit 1897 remains reasonably intact. In a later storyline, however, Gerard's haunting was apparently totally averted, so there would have been no need for Barnabas, Julia and Stokes to visit 1840 in the changed timeline, and therefore Gerard's ghost would be free to wreak havoc as it did originally...