Your best bet is to choose a server that is within the lightest bracket at prime time, which usually falls in the 8-10pm EST area or so. That is, unless you want to be on a crowded server :)
My server, Blackburrow, is almost *always* at Full Capacity status at primetime, at least all the times I've checked it it has been. The lighter load ones also usually tend to be the youngest servers, therefore you will have a better chance of newer players, with less of us grizzled insane month old vets around :D
EDIT - But if theres one thing that you shouldnt have to worry about now, is servers that are too underpopulated. Considering the game just came out and they are still adding new servers, I don't see this becoming a problem anytime in the near future.
Edited, Tue Dec 7 00:38:25 2004 by Stashy