My thoughts as below re casting (some picked from previous responds):
Belgarath - Harrison Ford (he has this twinkle in eye and u can just bung on some beard on him) or Sean Connery
Polgara - Miranda Richardson (From Merlin, she has this ageless quality in her face)
Poledra - Agree with someone re Sigourney Weaver, and also possibly Patricia Wettig (Holly from Brothers and Sisters)
Beldin - Danny Devito
The twins - Jim Broadbent
Belzedar - Ian Hurt
Note: Ian McKellan could do either Belgarath or Belzedar but he's too high-profile since LOTR.
Belgarion - Edward Speleers (as young Belgarion) and Ben Barnes (as older - the Mallorean series).
Durnik - Sam O'Neill
Ce'Nedra - Ginerva Molly or Mena Suvari
Silk - James Purefoy, Dennis/Randy Quiad
Barak - Robbie Coltrane or that gigantic guy from Troy that Hector killed
Hettar - Timothy Olyphant, Johnny Depp (dark brooding types who can look good in a topknot)
Mandorallen - Rufus Sewell, Matthew Fox,
Ledorellin - Orlando Bloom, James or Oliver Phelps (Weasley twins)
Asharak - Alan Rickman
Zakath - Antonio Banderas
Ctuchik - Hugo Weaving or Christopher Lee
Salmissra - Angelina Jolie (think of Grendel's mother from Beowulf, very predatory).
Cherek - John Rhys-Davies
Rodar - Paul Giacamotto
What do u guys reckon?