not so new boxer with a hardware (video card) issue.

[Codam]

Is this a common problem with people who run 5 accounts on 1 box?

I would be willing to bet that you would experience the same if you stressed the card by playing (solo) a more recent and more graphics-intensive game. The reason you dont get the artifacts when solo-playing WoW is that the card is not being stressed at all. There is in any case no logical reason why multiboxing would be the root of the problem.

My money is on defective memory on the card.