I'd cast that guy who played Gandalf in LotR to play Elminster, seeing as Elminster is basically Gandalf in the realms (well, maybe moer akin to a melding of Gandalf and Superman)
Elminster? I'd cast Liam Neeson. Elminster may look old, but he does not act his age physically. You need a younger actor, preferably between the age of 40-50.
The Simbul? Lucy Lawless. Seen her on Xena (bad girl try to do good ... well, I did say "try").
Alias? Sorry, but I'm not going to cast Jennifer Garner (of ABC's spy show Alias) but I'd go with Dina Meyer (Starship Trooper).
Dare to cast against type, people. You can't get the best actors if you don't let them stretch, and you can't make the next crop of the same if you don't let them try.
Actually after thinking about it for a few minutes this might work out a bit better.
Volo...Bruce Campbell
Manshoon...Tim Roth
King Obould...Mr. T
Szass Tam...Tom Baker
Vangerdahast...Dennis Farena
Storm Silverhand...Lady that played Major Kira from DS9
Voices for several dragons or other important creatures could be James Earl Jones, Patick Stewart, and John Travolta.