My local is very silty. A tackle shop I use stock some leads shaped like a 2 pound coin, they're excellent for this lake, because they seem to slow the whole set-up right down on entering the water. I also use pva bags with hooklength and weight inside, because of the large surface area created this also seems to keep things above the silt. I've recently read an article where a bloke swears by inline leads for silty bottoms, something about the way in which the lead falls through the water at an angle, because it is in direct contact with your mainline. I also like to use slightly longer hooklengths for my own piece of mind, usually something in the region of about 12".
As for the weight of leads, I don't own any over 2 oz, so anything under that does me.