Ahhhh, this old thread.
This idea has been going around for ages. All sorts of names have been bandied around for all sorts of different characters.
Some have been eminently sensible even divinely inspired such as Sean Connery as Belgarath. There is pretty much no one else who should play the part, let's be honest with ourselves.
Numerous names have been bandied about for Polgara, most of them are far too old to play the part. Remembering that Polgara is meant to be nearly as tall as a man, but she probably stopped aging before she hit thirty (talk about Oil of Ulan!).
Garion and Ce'Nedra will undoubtedly have to be done by unknowns. Frabnkly there is no one currently famous that really is anywhere decent enough to play it. The idea of having one actor to play them in the Belgariad and another in the Mallorean (supposing both were to be made) is just dumb frankly. It would ruin the continuity of the whole project. I would offer my services, but alas I fear I am getting too old now
As to Brak, well my choice has always been Gavin O'Herlihy. For those who are unaware, he played the part of Arik in Willow.... need I say more.
Silk is another strange one. I heard lots of interesting suggestions include one such as Eddie Murphy...
Yes I know.
But aside from that lunacy, he is a difficult one to cast. You need someone short, with a certain muscularity, and athleticism. It may end up being an unknown as well.
Of course this is most likely never to happen. But then we can dream.